[PATCH v5 00/13] Add video damage tracking
Alexander Graf
agraf at csgraf.de
Mon Aug 28 22:24:46 CEST 2023
On 28.08.23 19:54, Simon Glass wrote:
> Hi Alex,
>
> On Wed, 23 Aug 2023 at 02:56, Alexander Graf <agraf at csgraf.de> wrote:
>> Hey Simon,
>>
>> On 22.08.23 20:56, Simon Glass wrote:
>>> Hi Alex,
>>>
>>> On Tue, 22 Aug 2023 at 01:47, Alexander Graf <agraf at csgraf.de> wrote:
>>>> On 22.08.23 01:03, Simon Glass wrote:
>>>>> Hi Alex,
>>>>>
>>>>> On Mon, 21 Aug 2023 at 16:40, Alexander Graf <agraf at csgraf.de> wrote:
>>>>>> On 22.08.23 00:10, Simon Glass wrote:
>>>>>>> Hi Alex,
>>>>>>>
>>>>>>> On Mon, 21 Aug 2023 at 14:20, Alexander Graf <agraf at csgraf.de> wrote:
>>>>>>>> On 21.08.23 21:57, Simon Glass wrote:
>>>>>>>>> Hi Alex,
>>>>>>>>>
>>>>>>>>> On Mon, 21 Aug 2023 at 13:33, Alexander Graf <agraf at csgraf.de> wrote:
>>>>>>>>>> On 21.08.23 21:11, Simon Glass wrote:
>>>>>>>>>>> Hi Alper,
>>>>>>>>>>>
>>>>>>>>>>> On Mon, 21 Aug 2023 at 07:51, Alper Nebi Yasak <alpernebiyasak at gmail.com> wrote:
>>>>>>>>>>>> This is a rebase of Alexander Graf's video damage tracking series, with
>>>>>>>>>>>> some tests and other changes. The original cover letter is as follows:
>>>>>>>>>>>>
>>>>>>>>>>>>> This patch set speeds up graphics output on ARM by a factor of 60x.
>>>>>>>>>>>>>
>>>>>>>>>>>>> On most ARM SBCs, we keep the frame buffer in DRAM and map it as cached,
>>>>>>>>>>>>> but need it accessible by the display controller which reads directly
>>>>>>>>>>>>> from a later point of consistency. Hence, we flush the frame buffer to
>>>>>>>>>>>>> DRAM on every change. The full frame buffer.
>>>>>>>>>>> It should not, see below.
>>>>>>>>>>>
>>>>>>>>>>>>> Unfortunately, with the advent of 4k displays, we are seeing frame buffers
>>>>>>>>>>>>> that can take a while to flush out. This was reported by Da Xue with grub,
>>>>>>>>>>>>> which happily print 1000s of spaces on the screen to draw a menu. Every
>>>>>>>>>>>>> printed space triggers a cache flush.
>>>>>>>>>>> That is a bug somewhere in EFI.
>>>>>>>>>> Unfortunately not :). You may call it a bug in grub: It literally prints
>>>>>>>>>> over space characters for every character in its menu that it wants
>>>>>>>>>> cleared. On every text screen draw.
>>>>>>>>>>
>>>>>>>>>> This wouldn't be a big issue if we only flush the reactangle that gets
>>>>>>>>>> modified. But without this patch set, we're flushing the full DRAM
>>>>>>>>>> buffer on every u-boot text console character write, which means for
>>>>>>>>>> every character (as that's the only API UEFI has).
>>>>>>>>>>
>>>>>>>>>> As a nice side effect, we speed up the normal U-Boot text console as
>>>>>>>>>> well with this patch set, because even "normal" text prints that write
>>>>>>>>>> for example a single line of text on the screen today flush the full
>>>>>>>>>> frame buffer to DRAM.
>>>>>>>>> No, I mean that it is a bug that U-Boot (apparently) flushes the cache
>>>>>>>>> after every character. It doesn't do that for normal character output
>>>>>>>>> and I don't think it makes sense to do it for EFI either.
>>>>>>>> I see. Let's trace the calls:
>>>>>>>>
>>>>>>>> efi_cout_output_string()
>>>>>>>> -> fputs()
>>>>>>>> -> vidconsole_puts()
>>>>>>>> -> video_sync()
>>>>>>>> -> flush_dcache_range()
>>>>>>>>
>>>>>>>> Unfortunately grub abstracts character backends down to the "print
>>>>>>>> character" level, so it calls UEFI's sopisticated "output_string"
>>>>>>>> callback with single characters at a time, which means we do a full
>>>>>>>> dcache flush for every character that we print:
>>>>>>>>
>>>>>>>> https://git.savannah.gnu.org/cgit/grub.git/tree/grub-core/term/efi/console.c#n165
>>>>>>>>
>>>>>>>>
>>>>>>>>>>>>> This patch set implements the easiest mitigation against this problem:
>>>>>>>>>>>>> Damage tracking. We remember the lowest common denominator region that was
>>>>>>>>>>>>> touched since the last video_sync() call and only flush that. The most
>>>>>>>>>>>>> typical writer to the frame buffer is the video console, which always
>>>>>>>>>>>>> writes rectangles of characters on the screen and syncs afterwards.
>>>>>>>>>>>>>
>>>>>>>>>>>>> With this patch set applied, we reduce drawing a large grub menu (with
>>>>>>>>>>>>> serial console attached for size information) on an RK3399-ROC system
>>>>>>>>>>>>> at 1440p from 55 seconds to less than 1 second.
>>>>>>>>>>>>>
>>>>>>>>>>>>> Version 2 also implements VIDEO_COPY using this mechanism, reducing its
>>>>>>>>>>>>> overhead compared to before as well. So even x86 systems should be faster
>>>>>>>>>>>>> with this now :).
>>>>>>>>>>>>>
>>>>>>>>>>>>>
>>>>>>>>>>>>> Alternatives considered:
>>>>>>>>>>>>>
>>>>>>>>>>>>> 1) Lazy sync - Sandbox does this. It only calls video_sync(true) ever
>>>>>>>>>>>>> so often. We are missing timers to do this generically.
>>>>>>>>>>>>>
>>>>>>>>>>>>> 2) Double buffering - We could try to identify whether anything changed
>>>>>>>>>>>>> at all and only draw to the FB if it did. That would require
>>>>>>>>>>>>> maintaining a second buffer that we need to scan.
>>>>>>>>>>>>>
>>>>>>>>>>>>> 3) Text buffer - Maintain a buffer of all text printed on the screen with
>>>>>>>>>>>>> respective location. Don't write if the old and new character are
>>>>>>>>>>>>> identical. This would limit applicability to text only and is an
>>>>>>>>>>>>> optimization on top of this patch set.
>>>>>>>>>>>>>
>>>>>>>>>>>>> 4) Hash screen lines - Create a hash (sha256?) over every line when it
>>>>>>>>>>>>> changes. Only flush when it does. I'm not sure if this would waste
>>>>>>>>>>>>> more time, memory and cache than the current approach. It would make
>>>>>>>>>>>>> full screen updates much more expensive.
>>>>>>>>>>> 5) Fix the bug mentioned above?
>>>>>>>>>>>
>>>>>>>>>>>> Changes in v5:
>>>>>>>>>>>> - Add patch "video: test: Split copy frame buffer check into a function"
>>>>>>>>>>>> - Add patch "video: test: Support checking copy frame buffer contents"
>>>>>>>>>>>> - Add patch "video: test: Test partial updates of hardware frame buffer"
>>>>>>>>>>>> - Use xstart, ystart, xend, yend as names for damage region
>>>>>>>>>>>> - Document damage struct and fields in struct video_priv comment
>>>>>>>>>>>> - Return void from video_damage()
>>>>>>>>>>>> - Fix undeclared priv error in video_sync()
>>>>>>>>>>>> - Drop unused headers from video-uclass.c
>>>>>>>>>>>> - Use IS_ENABLED() instead of CONFIG_IS_ENABLED()
>>>>>>>>>>>> - Call video_damage() also in video_fill_part()
>>>>>>>>>>>> - Use met->baseline instead of priv->baseline
>>>>>>>>>>>> - Use fontdata->height/width instead of VIDEO_FONT_HEIGHT/WIDTH
>>>>>>>>>>>> - Update console_rotate.c video_damage() calls to pass video tests
>>>>>>>>>>>> - Remove mention about not having minimal damage for console_rotate.c
>>>>>>>>>>>> - Add patch "video: test: Test video damage tracking via vidconsole"
>>>>>>>>>>>> - Document new vdev field in struct efi_gop_obj comment
>>>>>>>>>>>> - Remove video_sync_copy() also from video_fill(), video_fill_part()
>>>>>>>>>>>> - Fix memmove() calls by removing the extra dev argument
>>>>>>>>>>>> - Call video_sync() before checking copy_fb in video tests
>>>>>>>>>>>> - Imply VIDEO_DAMAGE for video drivers instead of selecting it
>>>>>>>>>>>> - Imply VIDEO_DAMAGE also for VIDEO_TIDSS
>>>>>>>>>>>>
>>>>>>>>>>>> v4: https://lore.kernel.org/all/20230103215004.22646-1-agraf@csgraf.de/
>>>>>>>>>>>>
>>>>>>>>>>>> Changes in v4:
>>>>>>>>>>>> - Move damage clear to patch "dm: video: Add damage tracking API"
>>>>>>>>>>>> - Simplify first damage logic
>>>>>>>>>>>> - Remove VIDEO_DAMAGE default for ARM
>>>>>>>>>>>> - Skip damage on EfiBltVideoToBltBuffer
>>>>>>>>>>>> - Add patch "video: Always compile cache flushing code"
>>>>>>>>>>>> - Add patch "video: Enable VIDEO_DAMAGE for drivers that need it"
>>>>>>>>>>>>
>>>>>>>>>>>> v3: https://lore.kernel.org/all/20221230195828.88134-1-agraf@csgraf.de/
>>>>>>>>>>>>
>>>>>>>>>>>> Changes in v3:
>>>>>>>>>>>> - Adapt to always assume DM is used
>>>>>>>>>>>> - Adapt to always assume DM is used
>>>>>>>>>>>> - Make VIDEO_COPY always select VIDEO_DAMAGE
>>>>>>>>>>>>
>>>>>>>>>>>> v2: https://lore.kernel.org/all/20220609225921.62462-1-agraf@csgraf.de/
>>>>>>>>>>>>
>>>>>>>>>>>> Changes in v2:
>>>>>>>>>>>> - Remove ifdefs
>>>>>>>>>>>> - Fix ranges in truetype target
>>>>>>>>>>>> - Limit rotate to necessary damage
>>>>>>>>>>>> - Remove ifdefs from gop
>>>>>>>>>>>> - Fix dcache range; we were flushing too much before
>>>>>>>>>>>> - Add patch "video: Use VIDEO_DAMAGE for VIDEO_COPY"
>>>>>>>>>>>>
>>>>>>>>>>>> v1: https://lore.kernel.org/all/20220606234336.5021-1-agraf@csgraf.de/
>>>>>>>>>>>>
>>>>>>>>>>>> Alexander Graf (9):
>>>>>>>>>>>> dm: video: Add damage tracking API
>>>>>>>>>>>> dm: video: Add damage notification on display fills
>>>>>>>>>>>> vidconsole: Add damage notifications to all vidconsole drivers
>>>>>>>>>>>> video: Add damage notification on bmp display
>>>>>>>>>>>> efi_loader: GOP: Add damage notification on BLT
>>>>>>>>>>>> video: Only dcache flush damaged lines
>>>>>>>>>>>> video: Use VIDEO_DAMAGE for VIDEO_COPY
>>>>>>>>>>>> video: Always compile cache flushing code
>>>>>>>>>>>> video: Enable VIDEO_DAMAGE for drivers that need it
>>>>>>>>>>>>
>>>>>>>>>>>> Alper Nebi Yasak (4):
>>>>>>>>>>>> video: test: Split copy frame buffer check into a function
>>>>>>>>>>>> video: test: Support checking copy frame buffer contents
>>>>>>>>>>>> video: test: Test partial updates of hardware frame buffer
>>>>>>>>>>>> video: test: Test video damage tracking via vidconsole
>>>>>>>>>>>>
>>>>>>>>>>>> arch/arm/mach-omap2/omap3/Kconfig | 1 +
>>>>>>>>>>>> arch/arm/mach-sunxi/Kconfig | 1 +
>>>>>>>>>>>> drivers/video/Kconfig | 26 +++
>>>>>>>>>>>> drivers/video/console_normal.c | 27 ++--
>>>>>>>>>>>> drivers/video/console_rotate.c | 94 +++++++----
>>>>>>>>>>>> drivers/video/console_truetype.c | 37 +++--
>>>>>>>>>>>> drivers/video/exynos/Kconfig | 1 +
>>>>>>>>>>>> drivers/video/imx/Kconfig | 1 +
>>>>>>>>>>>> drivers/video/meson/Kconfig | 1 +
>>>>>>>>>>>> drivers/video/rockchip/Kconfig | 1 +
>>>>>>>>>>>> drivers/video/stm32/Kconfig | 1 +
>>>>>>>>>>>> drivers/video/tegra20/Kconfig | 1 +
>>>>>>>>>>>> drivers/video/tidss/Kconfig | 1 +
>>>>>>>>>>>> drivers/video/vidconsole-uclass.c | 16 --
>>>>>>>>>>>> drivers/video/video-uclass.c | 190 ++++++++++++----------
>>>>>>>>>>>> drivers/video/video_bmp.c | 7 +-
>>>>>>>>>>>> include/video.h | 59 +++----
>>>>>>>>>>>> include/video_console.h | 52 ------
>>>>>>>>>>>> lib/efi_loader/efi_gop.c | 7 +
>>>>>>>>>>>> test/dm/video.c | 256 ++++++++++++++++++++++++------
>>>>>>>>>>>> 20 files changed, 483 insertions(+), 297 deletions(-)
>>>>>>>>>>> It is good to see this tidied up into something that can be applied!
>>>>>>>>>>>
>>>>>>>>>>> I am unsure what is going on with the EFI performance, though. It
>>>>>>>>>>> should not flush the cache after every character, only after a new
>>>>>>>>>>> line. Is there something wrong in here? If so, we should fix that bug
>>>>>>>>>>> first and it should be patch 1 of this series.
>>>>>>>>>> Before I came up with this series, I was trying to identify the UEFI bug
>>>>>>>>>> in question as well, because intuition told me surely this is a bug in
>>>>>>>>>> UEFI :). Turns out it really isn't this time around.
>>>>>>>>> I don't mean a bug in UEFI, I mean a bug in U-Boot's EFI
>>>>>>>>> implementation. Where did you look for the bug?
>>>>>>>> The "real" bug is in grub. But given that it's reasonably simple to work
>>>>>>>> around in U-Boot and even with it "fixed" in grub we would still see
>>>>>>>> performance benefits from flushing only parts of the screen, I think
>>>>>>>> it's worth living with the grub deficiency.
>>>>>>> OK thanks for digging into it. I suggest we add a param to
>>>>>>> vidconsole_puts() to tell it whether to sync or not, then the EFI code
>>>>>>> can indicate this and try to be a bit smarter about it.
>>>>>> It doesn't know when to sync either. From its point of view, any
>>>>>> "console output" could be the last one. There is no API in UEFI that
>>>>>> says "please flush console output now".
>>>>> Yes, I understand. I was not suggesting we were missing an API. But
>>>>> some sort of heuristic would do, e.g. only flush on a newline, flush
>>>>> every 50 chars, etc.
>>>> I can't think of any heuristic that would reliably work. Relevant for
>>>> this conversation, UEFI provides 2 calls:
>>>>
>>>> * Write string to screen (efi_cout_output_string)
>>>> * Set text cursor position to X, Y (efi_cout_set_cursor_position)
>>>>
>>>> It's perfectly legal for a UEFI application to do something like
>>>>
>>>> efi_cout_set_cursor_position(10, 10);
>>>> efi_cout_output_string("f");
>>>> efi_cout_output_string("o");
>>>> efi_cout_output_string("o") ;
>>>>
>>>> to update contents of a virtual text box on the screen. Where in this
>>>> chain of events would we call video_sync(), but on every call to
>>>> efi_cout_output_string()?
>>> Actually U-Boot has the same problem, but we have managed to work out something.
>>
>> U-Boot as a code base has a much easier stance: It can add APIs when it
>> needs them in places that require them. With UEFI (as well as the U-Boot
>> native API), we're stuck with what's there.
>>
>> I also don't understand what you mean by "we have managed to work out
>> something". This patch set is not a UEFI fix - it fixes generic U-Boot
>> behavior and speeds up non-UEFI boots as well. The improvement there is
>> just not as impressive as with grub :).
> We are still not quite on the same page...
>
> U-Boot does have video_sync() but it doesn't know when to call it. If
> it does not call it, then any amount of single-threaded code can run
> after that, which may update the framebuffer. In other words, U-Boot
> is in exactly the same boat as UEFI. It has to decide whether to call
> video_sync() based on some sort of heuristic.
>
> That is the only point I am trying to make here. Does that make sense?
Oh, I thought you mentioned above that U-Boot is in a better spot or
"has it solved already". I agree - it's in the same boat and the only
safe thing it can really do today that is fully cross-platform
compatible is to call video_sync() after every character.
I don't understand what you mean by "any amount of single-threaded code
can run after that, which may update the framebuffer". Any framebuffer
modification is U-Boot internal code which then again can apply
video_sync() to tell the system "I want what I wrote to screen actually
be on screen now". I don't think that's necessarily bad design. A bit
clunky, but we're in a pre-boot environment after all.
Since we're aligned now: What exactly did you refer to with "but we have
managed to work out something"?
>
>>
>>> I do think it is still to flush the cache on every char. I suspect you
>>> will find that even a simple heuristic like I mentioned would be good
>>> enough.
>>>
>>> Also I notice that EFI calls notify? all the time, so U-Boot probably
>>> does have the ability to sync the video every 10ms if we wanted to.
>>
>> I fail to see how invalidating the frame buffer for the screen every
>> 10ms is any better than doing flush+invalidate operations only on screen
>> areas that changed? It's more fragile, more difficult to understand and
>> also significantly more expensive given that most of the time very
>> little on the screen actually changes.
> I am not suggesting it is better, at all. I am just trying to explain
> that the U-Boot EFI implementation should not be calling video_sync()
> after every character, before or after this series.
Ah, it doesn't :). It just calls the normal U-Boot "write character on
screen" function. What that does is up to U-Boot internals - and those
basically today dictate that something needs to call video_sync() to
make FB modifications actually pop up on screen.
>
>>
>>> It seems from this discussion that we have made great the enemy of the good.
>>
>> I agree. Damage tracking in this patch set is elegant, simple,
>> predictable, low overhead and basically just abstracts the video copy
>> code path to a generic solution. All while it pretty much solves the
>> issue for good. I don't understand what's not to like about it :)
> I think it is a really nice feature and I hope it can be applied soon.
Thanks a lot especially to Alper for picking it up. I had almost
forgotten about the patch set :)
Alex
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