Re: [PATCH 9/9] drm/virtio: Implement dumb_create_fbdev with GEM SHMEM helpers

[Thomas Zimmermann <tzimmermann@suse.de>]

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Hi

Am 08.03.22 um 20:37 schrieb Javier Martinez Canillas:
> On 3/3/22 21:58, Thomas Zimmermann wrote:
>> Implement struct drm_driver.dumb_create_fbdev with the helpers
>> provided by GEM SHMEM. Fbdev deferred I/O will now work without
>> an intermediate shadow buffer for mmap.
>>
>> As the virtio driver replaces several of the regular GEM SHMEM
>> functions with its own implementation, some additional code is
>> necessary make fbdev optimization work. Especially, the driver
>> has to provide buffer-object functions for fbdev. Add the hook
>> struct drm_driver.gem_create_object_fbdev, which is similar to
>> struct drm_driver.gem_create_object and allows for the creation
>> of dedicated fbdev buffer objects. Wire things up within GEM
>> SHMEM.
>>
>> Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
>> ---
>>   drivers/gpu/drm/drm_gem_shmem_helper.c  |  7 +++-
>>   drivers/gpu/drm/virtio/virtgpu_drv.c    |  2 +
>>   drivers/gpu/drm/virtio/virtgpu_drv.h    |  2 +
>>   drivers/gpu/drm/virtio/virtgpu_object.c | 54 +++++++++++++++++++++++--
>>   include/drm/drm_drv.h                   | 10 +++++
>>   5 files changed, 71 insertions(+), 4 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
>> index ab7cb7d896c3..225aa17895bd 100644
>> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
>> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
>> @@ -71,7 +71,12 @@ __drm_gem_shmem_create(struct drm_device *dev, size_t size, bool private, bool f
>>   
>>   	size = PAGE_ALIGN(size);
>>   
>> -	if (dev->driver->gem_create_object) {
>> +	if (dev->driver->gem_create_object_fbdev && fbdev) {
> 
> Same comment here to check if fbdev emulation is enabled or maybe is not
> worht it ? But I think this hints the compiler to optimize the if branch.
> 
> [snip]
> 
>> +}
>> +#else
>> +struct drm_gem_object *virtio_gpu_create_object_fbdev(struct drm_device *dev,
>> +						      size_t size)
>> +{
>> +	return ERR_PTR(-ENOSYS);
>> +}
> 
> As mentioned, I believe this should be ERR_PTR(-ENOTSUPP) instead.

I've been wondering about this as well. I finally went with the rules at 
[1].  All the variants of ENOTOP/ENOTSUPP seem to be for specific use 
cases, such as a certain feature is not implemented be a specific 
interface (e.g., sockets for EOPNOTSUPP).  ENOSYS is the only general 
error that indicates that an entire interface is missing. Even though 
checkpatch.pl warns that it's only for system calls.

Best regards
Thomas

[1] https://www.cs.helsinki.fi/linux/linux-kernel/2002-30/1135.html

> 
> Feel free to ignore all this nits if you consider that are not applicable.
> 
> Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
> 

-- 
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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