Hi Nicolas,

Am 17.06.21 um 11:36 schrieb nicolas saenz julienne:

On Sat, 2021-06-12 at 17:17 +0200, Stefan Wahren wrote:

...

Hi,

while testing the mainline kernel (arm64, defconfig) on Raspberry Pi 3 B Plus with Raspberry Pi OS - 64 bit, sometimes X doesn't start into desktop properly (unexpected and unusable login screen instead of auto login or mouse pointer is show shorty and than switch back to black screen in a loop). In that case dmesg shows the following:

[   74.737106] [drm:vc4_bo_create [vc4]] *ERROR* Failed to allocate from CMA: [   74.737558] vc4-drm soc:gpu: [drm]                            V3D:  28976kb BOs (10) [   74.737602] vc4-drm soc:gpu: [drm]                     V3D shader:     44kb BOs (11) [   74.737632] vc4-drm soc:gpu: [drm]                           dumb:   4564kb BOs (5) [   74.737664] vc4-drm soc:gpu: [drm]                         binner:  16384kb BOs (1) [   74.737697] vc4-drm soc:gpu: [drm]                total purged BO:      4kb BOs (1) [   74.739039] [drm:vc4_bo_create [vc4]] *ERROR* Failed to allocate from CMA: [   74.739466] vc4-drm soc:gpu: [drm]                            V3D:  28972kb BOs (9) [   74.739512] vc4-drm soc:gpu: [drm]                     V3D shader:     44kb BOs (11) [   74.739541] vc4-drm soc:gpu: [drm]                           dumb:   4564kb BOs (5) [   74.739570] vc4-drm soc:gpu: [drm]                         binner:  16384kb BOs (1) [   74.739602] vc4-drm soc:gpu: [drm]                total purged BO:      4kb BOs (1) [   74.740718] [drm:vc4_bo_create [vc4]] *ERROR* Failed to allocate from CMA: [   74.741138] vc4-drm soc:gpu: [drm]                            V3D:  28972kb BOs (9) [   74.741171] vc4-drm soc:gpu: [drm]                     V3D shader:     44kb BOs (11) [   74.741202] vc4-drm soc:gpu: [drm]                           dumb:   4564kb BOs (5) [   74.741231] vc4-drm soc:gpu: [drm]                         binner:  16384kb BOs (1) [   74.741263] vc4-drm soc:gpu: [drm]                total purged BO:      4kb BOs (1) ...

I have only seen this issue on arm64 with latest mainline kernel (5.13.0-rc5-00130-gf21b807c3cf8), but also with older kernel versions. So it's not a regression. It seems 64 bit needs more CMA.

In case X started properly i was also able to reproduce these errors above by dis- and reconneting HDMI.

So i increased CMA in bcm283x.dtsi and the problem disappeared:

iff --git a/arch/arm/boot/dts/bcm283x.dtsi b/arch/arm/boot/dts/bcm283x.dtsi index b83a864..d1304cb 100644 --- a/arch/arm/boot/dts/bcm283x.dtsi +++ b/arch/arm/boot/dts/bcm283x.dtsi @@ -37,7 +37,7 @@            cma: linux,cma {              compatible = "shared-dma-pool"; -            size = <0x4000000>; /* 64MB */ +            size = <0x6000000>; /* 96MB */              reusable;              linux,cma-default;          };

The questions are:

Is this the right way (tm) to fix this problem?

Frankly I don't know if there is a better way. IIRC opensuse and downstream use DT overlays to cater for this limitation. It seems reasonable to bump the value. But it'll be in detriment of users that don't care much for graphical interfaces. Nonetheless, I'm not familiar with how DRM handles CMA/DMA memory. So let me have a look at it. Maybe there is a SW fix. At first glance I'm surprised they can't defer to normal page allocations when CMA isn't capable of honoring the request (like the dma code does).

a compromise might be to increase the CMA size based on the SoC type (newer generations have more memory)

BCM2835 => 64 MB BCM2836, BCM2837 => 256 MB

...

And what is a sensible value (don't have a 4K display to test)?

The default for downstream is 256MB. But I've read discussions in the forum where people needed even more. IIUC it's use-case dependent, resolution is only one variable, you might then try to run a game and run out of memory there.

Sure this wasn't intend to make everybody happy. But i would expect to start X reliable at least.

Regards Stefan

Regards, Nicolas