Date:   Thu, 27 Oct 2022 13:08:24 +0200
From:   Thomas Zimmermann <tzimmermann@...e.de>
To:     Hector Martin <marcan@...can.st>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Javier Martinez Canillas <javierm@...hat.com>
Cc:     Pekka Paalanen <pekka.paalanen@...labora.com>,
        dri-devel@...ts.freedesktop.org, asahi@...ts.linux.dev,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH] drm/simpledrm: Only advertise formats that are supported

Hi

Am 27.10.22 um 12:13 schrieb Hector Martin:
> Until now, simpledrm unconditionally advertised all formats that can be
> supported natively as conversions. However, we don't actually have a
> full conversion matrix of helpers. Although the list is arguably
> provided to userspace in precedence order, userspace can pick something
> out-of-order (and thus break when it shouldn't), or simply only support
> a format that is unsupported (and thus think it can work, which results
> in the appearance of a hang as FB blits fail later on, instead of the
> initialization error you'd expect in this case).
> 
> Split up the format table into separate ones for each required subset,
> and then pick one based on the native format. Also remove the
> native<->conversion overlap check from the helper (which doesn't make
> sense any more, since the native format is advertised anyway and this
> way RGB565/RGB888 can share a format table), and instead print the same
> message in simpledrm when the native format is not one for which we have
> conversions at all.
> 
> This fixes a real user regression where the ?RGB2101010 support commit
> started advertising it unconditionally where not supported, and KWin
> decided to start to use it over the native format, but also the fixes
> the spurious RGB565/RGB888 formats which have been wrongly
> unconditionally advertised since the dawn of simpledrm.
> 
> Note: this patch is merged because splitting it into two patches, one
> for the helper and one for simpledrm, would regress at the midpoint
> regardless of the order. If simpledrm is changed first, that would break
> working conversions to RGB565/RGB888 (since those share a table that
> does not include the native formats). If the helper is changed first, it
> would start spuriously advertising all conversion formats when the
> native format doesn't have any supported conversions at all.
> 
> Acked-by: Pekka Paalanen <pekka.paalanen@...labora.com>
> Fixes: 6ea966fca084 ("drm/simpledrm: Add [AX]RGB2101010 formats")
> Fixes: 11e8f5fd223b ("drm: Add simpledrm driver")
> Cc: stable@...r.kernel.org
> Signed-off-by: Hector Martin <marcan@...can.st>
> ---
>   drivers/gpu/drm/drm_format_helper.c | 15 -------
>   drivers/gpu/drm/tiny/simpledrm.c    | 62 +++++++++++++++++++++++++----

We currently have two DRM drivers that call drm_fb_build_fourcc_list(): 
simpledrm and ofdrm. I've been very careful to keep the format selection 
in sync between them. (That's the reason why the helper exists at all.) 
If the drivers start to use different logic, it will only become more 
chaotic.

The format array of ofdrm is at [1]. At a minimum, ofdrm should get the 
same fix as simpledrm.

[1] 
https://cgit.freedesktop.org/drm/drm-tip/tree/drivers/gpu/drm/tiny/ofdrm.c#n760

>   2 files changed, 55 insertions(+), 22 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_format_helper.c b/drivers/gpu/drm/drm_format_helper.c
> index e2f76621453c..c60c13f3a872 100644
> --- a/drivers/gpu/drm/drm_format_helper.c
> +++ b/drivers/gpu/drm/drm_format_helper.c
> @@ -864,20 +864,6 @@ size_t drm_fb_build_fourcc_list(struct drm_device *dev,
>   		++fourccs;
>   	}
>   
> -	/*
> -	 * The plane's atomic_update helper converts the framebuffer's color format
> -	 * to a native format when copying to device memory.
> -	 *
> -	 * If there is not a single format supported by both, device and
> -	 * driver, the native formats are likely not supported by the conversion
> -	 * helpers. Therefore *only* support the native formats and add a
> -	 * conversion helper ASAP.
> -	 */
> -	if (!found_native) {
> -		drm_warn(dev, "Format conversion helpers required to add extra formats.\n");
> -		goto out;
> -	}
> -
>   	/*
>   	 * The extra formats, emulated by the driver, go second.
>   	 */
> @@ -898,7 +884,6 @@ size_t drm_fb_build_fourcc_list(struct drm_device *dev,
>   		++fourccs;
>   	}
>   
> -out:
>   	return fourccs - fourccs_out;
>   }
>   EXPORT_SYMBOL(drm_fb_build_fourcc_list);
> diff --git a/drivers/gpu/drm/tiny/simpledrm.c b/drivers/gpu/drm/tiny/simpledrm.c
> index 18489779fb8a..1257411f3d44 100644
> --- a/drivers/gpu/drm/tiny/simpledrm.c
> +++ b/drivers/gpu/drm/tiny/simpledrm.c
> @@ -446,22 +446,48 @@ static int simpledrm_device_init_regulators(struct simpledrm_device *sdev)
>    */
>   
>   /*
> - * Support all formats of simplefb and maybe more; in order
> - * of preference. The display's update function will do any
> + * Support the subset of formats that we have conversion helpers for,
> + * in order of preference. The display's update function will do any
>    * conversion necessary.
>    *
>    * TODO: Add blit helpers for remaining formats and uncomment
>    *       constants.
>    */
> -static const uint32_t simpledrm_primary_plane_formats[] = {
> +
> +/*
> + * Supported conversions to RGB565 and RGB888:
> + *   from [AX]RGB8888
> + */
> +static const uint32_t simpledrm_primary_plane_formats_base[] = {
> +	DRM_FORMAT_XRGB8888,
> +	DRM_FORMAT_ARGB8888,
> +};
> +
> +/*
> + * Supported conversions to [AX]RGB8888:
> + *   A/X variants (no-op)
> + *   from RGB565
> + *   from RGB888
> + */
> +static const uint32_t simpledrm_primary_plane_formats_xrgb8888[] = {
>   	DRM_FORMAT_XRGB8888,
>   	DRM_FORMAT_ARGB8888,
> +	DRM_FORMAT_RGB888,
>   	DRM_FORMAT_RGB565,
>   	//DRM_FORMAT_XRGB1555,
>   	//DRM_FORMAT_ARGB1555,
> -	DRM_FORMAT_RGB888,
> +};
> +
> +/*
> + * Supported conversions to [AX]RGB2101010:
> + *   A/X variants (no-op)
> + *   from [AX]RGB8888
> + */
> +static const uint32_t simpledrm_primary_plane_formats_xrgb2101010[] = {
>   	DRM_FORMAT_XRGB2101010,
>   	DRM_FORMAT_ARGB2101010,
> +	DRM_FORMAT_XRGB8888,
> +	DRM_FORMAT_ARGB8888,
>   };
>   
>   static const uint64_t simpledrm_primary_plane_format_modifiers[] = {
> @@ -642,7 +668,8 @@ static struct simpledrm_device *simpledrm_device_create(struct drm_driver *drv,
>   	struct drm_encoder *encoder;
>   	struct drm_connector *connector;
>   	unsigned long max_width, max_height;
> -	size_t nformats;
> +	const uint32_t *conv_formats;
> +	size_t conv_nformats, nformats;
>   	int ret;
>   
>   	sdev = devm_drm_dev_alloc(&pdev->dev, drv, struct simpledrm_device, dev);
> @@ -755,10 +782,31 @@ static struct simpledrm_device *simpledrm_device_create(struct drm_driver *drv,
>   	dev->mode_config.funcs = &simpledrm_mode_config_funcs;
>   
>   	/* Primary plane */
> +	switch (format->format) {

I trust you when you say that <native>->XRGB8888 is not enough. But 
although I've read your replies, I still don't understand why this 
switch is necessary.

Why don't we call drm_fb_build_fourcc_list() with the native 
format/formats and let it append a number of formats, such as adding 
XRGB888, adding ARGB8888 if necessary, adding ARGB2101010 if necessary. 
Each with a elaborate comment why and which userspace needs the format. (?)

Best regards
Thomas

> +	case DRM_FORMAT_RGB565:
> +	case DRM_FORMAT_RGB888:
> +		conv_formats = simpledrm_primary_plane_formats_base;
> +		conv_nformats = ARRAY_SIZE(simpledrm_primary_plane_formats_base);
> +		break;
> +	case DRM_FORMAT_XRGB8888:
> +	case DRM_FORMAT_ARGB8888:
> +		conv_formats = simpledrm_primary_plane_formats_xrgb8888;
> +		conv_nformats = ARRAY_SIZE(simpledrm_primary_plane_formats_xrgb8888);
> +		break;
> +	case DRM_FORMAT_XRGB2101010:
> +	case DRM_FORMAT_ARGB2101010:
> +		conv_formats = simpledrm_primary_plane_formats_xrgb2101010;
> +		conv_nformats = ARRAY_SIZE(simpledrm_primary_plane_formats_xrgb2101010);
> +		break;
> +	default:
> +		conv_formats = NULL;
> +		conv_nformats = 0;
> +		drm_warn(dev, "Format conversion helpers required to add extra formats.\n");
> +		break;
> +	}
>   
>   	nformats = drm_fb_build_fourcc_list(dev, &format->format, 1,
> -					    simpledrm_primary_plane_formats,
> -					    ARRAY_SIZE(simpledrm_primary_plane_formats),
> +					    conv_formats, conv_nformats,
>   					    sdev->formats, ARRAY_SIZE(sdev->formats));
>   
>   	primary_plane = &sdev->primary_plane;

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Ivo Totev

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