linux-kernel - Re: [PATCH v10 06/11] drm/shmem-helper: Don't use vmap_use

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Date:   Fri, 17 Feb 2023 13:41:20 +0100
From:   Thomas Zimmermann <tzimmermann@...e.de>
To:     Dmitry Osipenko <dmitry.osipenko@...labora.com>,
        David Airlie <airlied@...il.com>,
        Gerd Hoffmann <kraxel@...hat.com>,
        Gurchetan Singh <gurchetansingh@...omium.org>,
        Chia-I Wu <olvaffe@...il.com>, Daniel Vetter <daniel@...ll.ch>,
        Daniel Almeida <daniel.almeida@...labora.com>,
        Gustavo Padovan <gustavo.padovan@...labora.com>,
        Daniel Stone <daniel@...ishbar.org>,
        Tomeu Vizoso <tomeu.vizoso@...labora.com>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Rob Clark <robdclark@...il.com>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Christian König <christian.koenig@....com>,
        Qiang Yu <yuq825@...il.com>,
        Steven Price <steven.price@....com>,
        Alyssa Rosenzweig <alyssa.rosenzweig@...labora.com>,
        Rob Herring <robh@...nel.org>, Sean Paul <sean@...rly.run>,
        Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
        Abhinav Kumar <quic_abhinavk@...cinc.com>
Cc:     kernel@...labora.com, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH v10 06/11] drm/shmem-helper: Don't use vmap_use_count for
 dma-bufs

Hi

Am 08.01.23 um 22:04 schrieb Dmitry Osipenko:
> DMA-buf core has its own refcounting of vmaps, use it instead of drm-shmem
> counting. This change prepares drm-shmem for addition of memory shrinker
> support where drm-shmem will use a single dma-buf reservation lock for
> all operations performed over dma-bufs.
> 
> Signed-off-by: Dmitry Osipenko <dmitry.osipenko@...labora.com>

Reviewed-by: Thomas Zimmermann <tzimmermann@...e.de>

with my comments below considered.

> ---
>   drivers/gpu/drm/drm_gem_shmem_helper.c | 35 +++++++++++++++-----------
>   1 file changed, 20 insertions(+), 15 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
> index 5006f7da7f2d..1392cbd3cc02 100644
> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
> @@ -301,24 +301,22 @@ static int drm_gem_shmem_vmap_locked(struct drm_gem_shmem_object *shmem,
>   	struct drm_gem_object *obj = &shmem->base;
>   	int ret = 0;
>   
> -	if (shmem->vmap_use_count++ > 0) {
> -		iosys_map_set_vaddr(map, shmem->vaddr);
> -		return 0;
> -	}
> -
>   	if (obj->import_attach) {
>   		ret = dma_buf_vmap(obj->import_attach->dmabuf, map);
>   		if (!ret) {
>   			if (drm_WARN_ON(obj->dev, map->is_iomem)) {

I'm sure that I added this line at some point. But I'm now wondering why 
we're testing this flag. Everything that uses the mapped buffer should 
by agnostic to is_iomem. IIRC the only reason for this test is is that 
we're setting shmem->vaddr to the returned map->vaddr. Now that the code 
is gone, we can also remove that whole branch.

>   				dma_buf_vunmap(obj->import_attach->dmabuf, map);
> -				ret = -EIO;
> -				goto err_put_pages;
> +				return -EIO;
>   			}
> -			shmem->vaddr = map->vaddr;
>   		}
>   	} else {
>   		pgprot_t prot = PAGE_KERNEL;
>   
> +		if (shmem->vmap_use_count++ > 0) {
> +			iosys_map_set_vaddr(map, shmem->vaddr);
> +			return 0;
> +		}
> +
>   		ret = drm_gem_shmem_get_pages(shmem);
>   		if (ret)
>   			goto err_zero_use;
> @@ -384,15 +382,15 @@ static void drm_gem_shmem_vunmap_locked(struct drm_gem_shmem_object *shmem,
>   {
>   	struct drm_gem_object *obj = &shmem->base;
>   
> -	if (drm_WARN_ON_ONCE(obj->dev, !shmem->vmap_use_count))
> -		return;
> -
> -	if (--shmem->vmap_use_count > 0)
> -		return;
> -
>   	if (obj->import_attach) {
>   		dma_buf_vunmap(obj->import_attach->dmabuf, map);
>   	} else {
> +		if (drm_WARN_ON_ONCE(obj->dev, !shmem->vmap_use_count))
> +			return;
> +
> +		if (--shmem->vmap_use_count > 0)
> +			return;
> +
>   		vunmap(shmem->vaddr);
>   		drm_gem_shmem_put_pages(shmem);
>   	}
> @@ -660,7 +658,14 @@ void drm_gem_shmem_print_info(const struct drm_gem_shmem_object *shmem,
>   			      struct drm_printer *p, unsigned int indent)
>   {
>   	drm_printf_indent(p, indent, "pages_use_count=%u\n", shmem->pages_use_count);
> -	drm_printf_indent(p, indent, "vmap_use_count=%u\n", shmem->vmap_use_count);
> +
> +	if (shmem->base.import_attach)
> +		drm_printf_indent(p, indent, "vmap_use_count=%u\n",
> +				  shmem->base.dma_buf->vmapping_counter);

This is not vmap_use_count and the best solution is to add a print_info 
callback to dma_bufs. So Maybe simply ignore imported buffers here.

Best regards
Thomas

> +	else
> +		drm_printf_indent(p, indent, "vmap_use_count=%u\n",
> +				  shmem->vmap_use_count);
> +
>   	drm_printf_indent(p, indent, "vaddr=%p\n", shmem->vaddr);
>   }
>   EXPORT_SYMBOL(drm_gem_shmem_print_info);

-- 
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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