GPU acceleration with SPI displays (STM32MP157 with Vivante GPU)

[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.
>
> 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
>