[PATCH v5 00/13] Add video damage tracking
Simon Glass
sjg at chromium.org
Tue Aug 22 00:10:43 CEST 2023
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.
Regards,
Simon
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