[PATCH v5 00/13] Add video damage tracking

Mon Aug 21 21:57:12 CEST 2023

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.

>
>
> >
> >>> 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?

Regards,
Simon