GPU acceleration with SPI displays (STM32MP157 with Vivante GPU)

[Josef Luštický]

On Tue, Aug 5, 2025 at 10:19 AM Lucas Stach <l.stach at pengutronix.de> wrote:
>
> Am Dienstag, dem 05.08.2025 um 08:46 +0200 schrieb Josef Luštický:
> > On Mon, Aug 4, 2025 at 11:59 AM Lucas Stach <l.stach at pengutronix.de> wrote:
> > >
> > > Hi Josef,
> > Hello Lucas, thank you for answering.
> > >
> > > Am Freitag, dem 01.08.2025 um 13:06 +0200 schrieb Josef Luštický:
> > > > Hello,
> > > > I am trying to get GPU acceleration to work with SPI display using
> > > > ili9225_dri.so,
> > > > and I use kmscube to test.
> > > > The SoC is STM32MP157c with Vivante GC400 GPU and I use Mesa3D 24.0.9
> > > > (rootfs generated with Buildroot 2025.02.3) and kernel 5.10.176 (from
> > > > STM).
> > > >
> > > > I also have the same system with MIPI Display attached.
> > > > While kmscube renders at 60 fps with the MIPI Display system, I get
> > > > only 11 fps with the SPI display. Apart from that both systems are the
> > > > same.
> > >
> > > The question is what can you expect from a SPI display? SPI is not a
> > > great interface for isochronous traffic with quite high bandwidth
> > > demands. Even at really low resolutions you need a lot of bandwidth to
> > > update a display. What is the resolution and color depth of you
> > > display? How fast is your SPI connection?
> > The display is 320x480 with 16bpp, so the screen is 307200 bytes.
> > The SPI clock is set to 50MHz in device-tree and I also tried 100MHz
> > with no change.
> > From the clock tree configuration, it seems the SPI peripheral gets 64MHz,
> > but I'll check what's the real frequency of SPI CLK output.
>
> If this is a standard SPI connection with a single bit per clock
> transfer rate, 50 MHz SPI clock rate works out to ~20 frames per
> second, limited by pure transfer time. This means no other overhead
> like SPI message setup in the host controller and userspace waiting for
> flip completion or anything like that.
I measured the SPI CLK frequency and it turns out it was 32MHz.
After increasing to 50MHz I am now getting 17 fps with the kmscube demo.
With 32MHz the theoretical maximum is 32000000 / (320*480*16) = 13 fps,
while with 50MHZ it is 50000000 / (320*480*16) = 20.3 fps.

So you're right Lucas,
Thank you
Josef
>
> Given that, your measured 11 fps doesn't sound that much off from what
> you could expect from such a setup.
>
> Regards,
> Lucas
>
> > >
> > > You might run modetest -v to get a reference point on how fast the
> > > display can be updated without any GPU rendering going on.
> > >
> > > Regards,
> > > Lucas
> > >
> > > >
> > > > I checked the output of kmscube and it prints the exact same extensions on both,
> > > > the only difference is the framerate.
> > > > I also checked output of kmscube's /proc/PID/maps and the only
> > > > difference is that with MIPI Display it loads /usr/lib/dri/stm_dri.so
> > > > while with the SPI Display it loads /usr/lib/dri/ili9225_dri.so,
> > > > but I see those files are the same (hardlinks).
> > > > Both open /dev/dri/renderD128 (GPU).
> > > >
> > > > Why am I getting just 11 fps on the system with SPI Display?
> > > > Is there some architectural limitation, either in Mesa, kernel, or HW-related?
> > > > I suppose both setups use kernel's KMS as renderer only (kmsro), thus
> > > > it should render at the same framerate.
> > > > The difference in HW is the peripheral the displays use - LTDC
> > > > (stm_dri.so) vs. SPI (eventually ili9225_dri.so).
> > > > Why do I see no /usr/lib/etnaviv_dri.so loaded?
> > > >
> > > > The MIPI Display is attached to the LTDC (LCD Timing Display
> > > > Controller) peripheral of the STM SoC and the display driver is
> > > > drivers/gpu/drm/panel/panel-sitronix-st7703.c (DRM panel).
> > > >
> > > > The SPI display driver uses the kernel's TinyDRM API and its driver is
> > > > drivers/gpu/drm/tiny/ili9225.c.
> > > >
> > > > I also tried softpipe (disabled GPU in device-tree) and it renders 7
> > > > fps on both systems.
> > > >
> > > > I attach the output from kmscube on the SPI Display system (the MIPI
> > > > Display is the same except framerate is 60 fps.)
> > > >
> > > > Kind regards
> > > > Josef Lusticky
> > >
>