Re: [PATCH v3 1/2] drm: Add library for shmem backed GEM objects

[David Lechner <david@lechnology.com>]

On 09/11/2018 07:43 AM, Noralf Trønnes wrote:
> This adds a library for shmem backed GEM objects with the necessary
> drm_driver callbacks.
> 
> Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
> ---
> 

...

> +static int drm_gem_shmem_vmap_locked(struct drm_gem_shmem_object *shmem)
> +{
> +	struct drm_gem_object *obj = &shmem->base;
> +	int ret;
> +
> +	if (shmem->vmap_use_count++ > 0)
> +		return 0;
> +
> +	ret = drm_gem_shmem_get_pages(shmem);
> +	if (ret)
> +		goto err_zero_use;
> +
> +	if (obj->import_attach) {
> +		shmem->vaddr = dma_buf_vmap(obj->import_attach->dmabuf);
> +	} else {
> +		pgprot_t prot;
> +
> +		switch (shmem->cache_mode) {
> +		case DRM_GEM_SHMEM_BO_UNKNOWN:
> +			DRM_DEBUG_KMS("Can't vmap cache mode is unknown\n");
> +			ret = -EINVAL;
> +			goto err_put_pages;
> +
> +		case DRM_GEM_SHMEM_BO_WRITECOMBINED:
> +			prot = pgprot_writecombine(PAGE_KERNEL);
> +			break;
> +
> +		case DRM_GEM_SHMEM_BO_UNCACHED:
> +			prot = pgprot_noncached(PAGE_KERNEL);
> +			break;
> +
> +		case DRM_GEM_SHMEM_BO_CACHED:
> +			prot = PAGE_KERNEL;
> +			break;
> +		}
> +
> +		shmem->vaddr = vmap(shmem->pages, obj->size >> PAGE_SHIFT, VM_MAP, prot);

I get a gcc warning here:

/home/david/work/ev3dev2/ev3dev-kernel/drivers/gpu/drm/drm_gem_shmem_helper.c:220:18: warning: ‘prot’ may be used uninitialized in this function [-Wmaybe-uninitialized]
    shmem->vaddr = vmap(shmem->pages, obj->size >> PAGE_SHIFT, VM_MAP, prot);
                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/david/work/ev3dev2/ev3dev-kernel/drivers/gpu/drm/drm_gem_shmem_helper.c:199:12: note: ‘prot’ was declared here
    pgprot_t prot;
             ^~~~

---

And since I am making a comment anyway, it is not clear to me
what BO means in the enum names. I didn't see any hints in the
doc comments either.


> +	}
> +
> +	if (!shmem->vaddr) {
> +		DRM_DEBUG_KMS("Failed to vmap pages\n");
> +		ret = -ENOMEM;
> +		goto err_put_pages;
> +	}
> +
> +	return 0;
> +
> +err_put_pages:
> +	drm_gem_shmem_put_pages(shmem);
> +err_zero_use:
> +	shmem->vmap_use_count = 0;
> +
> +	return ret;
> +}
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