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Subject: Re: [PATCH v2 1/2] drm/shmem: add support for per object caching
 attributes
From:   Thomas Zimmermann <tzimmermann@suse.de>
To:     Gerd Hoffmann <kraxel@redhat.com>, dri-devel@lists.freedesktop.org
Cc:     David Airlie <airlied@linux.ie>,
        open list <linux-kernel@vger.kernel.org>,
        gurchetansingh@chromium.org
References: <20191211081810.20079-1-kraxel@redhat.com>
 <20191211081810.20079-2-kraxel@redhat.com>
 <0b64e917-48f7-487e-9335-2838b6c62808@suse.de>
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Message-ID: <ed9142da-ce10-7df2-8a85-ba9ad0c26551@suse.de>
Date:   Wed, 11 Dec 2019 11:07:25 +0100
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From: Thomas Zimmermann <tzimmermann@suse.de>
To: Gerd Hoffmann <kraxel@redhat.com>, dri-devel@lists.freedesktop.org
Cc: David Airlie <airlied@linux.ie>, open list
 <linux-kernel@vger.kernel.org>, gurchetansingh@chromium.org
Message-ID: <ed9142da-ce10-7df2-8a85-ba9ad0c26551@suse.de>
Subject: Re: [PATCH v2 1/2] drm/shmem: add support for per object caching
 attributes
References: <20191211081810.20079-1-kraxel@redhat.com>
 <20191211081810.20079-2-kraxel@redhat.com>
 <0b64e917-48f7-487e-9335-2838b6c62808@suse.de>
In-Reply-To: <0b64e917-48f7-487e-9335-2838b6c62808@suse.de>

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Am 11.12.19 um 10:58 schrieb Thomas Zimmermann:
> Hi Gerd
>=20
> Am 11.12.19 um 09:18 schrieb Gerd Hoffmann:
>> Add caching field to drm_gem_shmem_object to specify the cachine
>> attributes for mappings.  Add helper function to tweak pgprot
>> accordingly.  Switch vmap and mmap functions to the new helper.
>>
>> Set caching to write-combine when creating the object so behavior
>> doesn't change by default.  Drivers can override that later if
>> needed.
>>
>> Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
>=20
> If you want to merge this patch, you have my
>=20
> Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
>=20
> Please see my comment below.
>=20
>> ---
>>  include/drm/drm_gem_shmem_helper.h     | 12 ++++++++++++
>>  drivers/gpu/drm/drm_gem_shmem_helper.c | 24 +++++++++++++++++++++---
>>  2 files changed, 33 insertions(+), 3 deletions(-)
>>
>> diff --git a/include/drm/drm_gem_shmem_helper.h b/include/drm/drm_gem_=
shmem_helper.h
>> index 6748379a0b44..9d6e02c6205f 100644
>> --- a/include/drm/drm_gem_shmem_helper.h
>> +++ b/include/drm/drm_gem_shmem_helper.h
>> @@ -17,6 +17,11 @@ struct drm_mode_create_dumb;
>>  struct drm_printer;
>>  struct sg_table;
>> =20
>> +enum drm_gem_shmem_caching {
>> +	DRM_GEM_SHMEM_CACHED =3D 1,
>> +	DRM_GEM_SHMEM_WC,
>> +};
>> +
>>  /**
>>   * struct drm_gem_shmem_object - GEM object backed by shmem
>>   */
>> @@ -83,6 +88,11 @@ struct drm_gem_shmem_object {
>>  	 * The address are un-mapped when the count reaches zero.
>>  	 */
>>  	unsigned int vmap_use_count;
>> +
>> +	/**
>> +	 * @caching: caching attributes for mappings.
>> +	 */
>> +	enum drm_gem_shmem_caching caching;
>>  };
>> =20
>>  #define to_drm_gem_shmem_obj(obj) \
>> @@ -130,6 +140,8 @@ drm_gem_shmem_prime_import_sg_table(struct drm_dev=
ice *dev,
>> =20
>>  struct sg_table *drm_gem_shmem_get_pages_sgt(struct drm_gem_object *o=
bj);
>> =20
>> +pgprot_t drm_gem_shmem_caching(struct drm_gem_shmem_object *shmem, pg=
prot_t prot);
>> +
>>  /**
>>   * DRM_GEM_SHMEM_DRIVER_OPS - Default shmem GEM operations
>>   *
>> diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/=
drm_gem_shmem_helper.c
>> index a421a2eed48a..5bb94e130a50 100644
>> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
>> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
>> @@ -76,6 +76,7 @@ struct drm_gem_shmem_object *drm_gem_shmem_create(st=
ruct drm_device *dev, size_t
>>  	mutex_init(&shmem->pages_lock);
>>  	mutex_init(&shmem->vmap_lock);
>>  	INIT_LIST_HEAD(&shmem->madv_list);
>> +	shmem->caching =3D DRM_GEM_SHMEM_WC;
>> =20
>>  	/*
>>  	 * Our buffers are kept pinned, so allocating them
>> @@ -256,9 +257,11 @@ static void *drm_gem_shmem_vmap_locked(struct drm=
_gem_shmem_object *shmem)
>> =20
>>  	if (obj->import_attach)
>>  		shmem->vaddr =3D dma_buf_vmap(obj->import_attach->dmabuf);
>> -	else
>> +	else {
>> +		pgprot_t prot =3D drm_gem_shmem_caching(shmem, PAGE_KERNEL);
>>  		shmem->vaddr =3D vmap(shmem->pages, obj->size >> PAGE_SHIFT,
>> -				    VM_MAP, pgprot_writecombine(PAGE_KERNEL));
>> +				    VM_MAP, prot);
>> +	}
>> =20
>>  	if (!shmem->vaddr) {
>>  		DRM_DEBUG_KMS("Failed to vmap pages\n");
>> @@ -540,7 +543,8 @@ int drm_gem_shmem_mmap(struct drm_gem_object *obj,=
 struct vm_area_struct *vma)
>>  	}
>> =20
>>  	vma->vm_flags |=3D VM_MIXEDMAP | VM_DONTEXPAND;
>> -	vma->vm_page_prot =3D pgprot_writecombine(vm_get_page_prot(vma->vm_f=
lags));
>> +	vma->vm_page_prot =3D vm_get_page_prot(vma->vm_flags);
>> +	vma->vm_page_prot =3D drm_gem_shmem_caching(shmem, vma->vm_page_prot=
);
>>  	vma->vm_page_prot =3D pgprot_decrypted(vma->vm_page_prot);
>>  	vma->vm_ops =3D &drm_gem_shmem_vm_ops;
>> =20
>> @@ -683,3 +687,17 @@ drm_gem_shmem_prime_import_sg_table(struct drm_de=
vice *dev,
>>  	return ERR_PTR(ret);
>>  }
>>  EXPORT_SYMBOL_GPL(drm_gem_shmem_prime_import_sg_table);
>> +
>> +pgprot_t drm_gem_shmem_caching(struct drm_gem_shmem_object *shmem, pg=
prot_t prot)
>> +{
>> +	switch (shmem->caching) {
>> +	case DRM_GEM_SHMEM_CACHED:
>> +		return prot;
>> +	case DRM_GEM_SHMEM_WC:
>> +		return pgprot_writecombine(prot);
>> +	default:
>> +		WARN_ON_ONCE(1);
>> +		return prot;
>> +	}
>> +}
>> +EXPORT_SYMBOL_GPL(drm_gem_shmem_caching);
>=20
> Two reason why I'd reconsider this design.
>=20
> I don't like switch statements new the bottom of the call graph. The
> code ends up with default warnings, such as this one.
>=20
> Udl has different caching flags for imported and 'native' buffers. This=

> would require a new constant and additional code here.
>=20
> What do you think about turning this function into a callback in struct=

> shmem_funcs? The default implementation would be for WC, virtio would
> use CACHED. The individual implementations could still be located in th=
e
> shmem code. Udl would later provide its own code.

On a second thought, all this might be over-engineered and v1 of the
patchset was the correct approach. You can add my

Acked-by: Thomas Zimmermann <tzimmermann@suse.de>

if you prefer to merge v1.

>=20
> Best regards
> Thomas
>=20
>>
>=20

--=20
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 N=C3=BCrnberg, Germany
(HRB 36809, AG N=C3=BCrnberg)
Gesch=C3=A4ftsf=C3=BChrer: Felix Imend=C3=B6rffer


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