Re: [PATCH v3] drivers: introduce and use WANT_DMA_CMA for soft dependencies on DMA_CMA

[David Hildenbrand <david@redhat.com>]

On 09.04.21 15:35, Arnd Bergmann wrote:
> On Fri, Apr 9, 2021 at 1:21 PM David Hildenbrand <david@redhat.com> wrote:
>>
>> Random drivers should not override a user configuration of core knobs
>> (e.g., CONFIG_DMA_CMA=n). Applicable drivers would like to use DMA_CMA,
>> which depends on CMA, if possible; however, these drivers also have to
>> tolerate if DMA_CMA is not available/functioning, for example, if no CMA
>> area for DMA_CMA use has been setup via "cma=X". In the worst case, the
>> driver cannot do it's job properly in some configurations.
>>
>> For example, commit 63f5677544b3 ("drm/etnaviv: select CMA and DMA_CMA if
>> available") documents
>>          While this is no build dependency, etnaviv will only work correctly
>>          on most systems if CMA and DMA_CMA are enabled. Select both options
>>          if available to avoid users ending up with a non-working GPU due to
>>          a lacking kernel config.
>> So etnaviv really wants to have DMA_CMA, however, can deal with some cases
>> where it is not available.
>>
>> Let's introduce WANT_DMA_CMA and use it in most cases where drivers
>> select CMA/DMA_CMA, or depend on DMA_CMA (in a wrong way via CMA because
>> of recursive dependency issues).
>>
>> We'll assume that any driver that selects DRM_GEM_CMA_HELPER or
>> DRM_KMS_CMA_HELPER would like to use DMA_CMA if possible.
>>
>> With this change, distributions can disable CONFIG_CMA or
>> CONFIG_DMA_CMA, without it silently getting enabled again by random
>> drivers. Also, we'll now automatically try to enabled both, CONFIG_CMA
>> and CONFIG_DMA_CMA if they are unspecified and any driver is around that
>> selects WANT_DMA_CMA -- also implicitly via DRM_GEM_CMA_HELPER or
>> DRM_KMS_CMA_HELPER.
>>
>> For example, if any driver selects WANT_DMA_CMA and we do a
>> "make olddefconfig":
>>
>> 1. With "# CONFIG_CMA is not set" and no specification of
>>     "CONFIG_DMA_CMA"
>>
>> -> CONFIG_DMA_CMA won't be part of .config
>>
>> 2. With no specification of CONFIG_CMA or CONFIG_DMA_CMA
>>
>> Contiguous Memory Allocator (CMA) [Y/n/?] (NEW)
>> DMA Contiguous Memory Allocator (DMA_CMA) [Y/n/?] (NEW)
>>
>> 3. With "# CONFIG_CMA is not set" and "# CONFIG_DMA_CMA is not set"
>>
>> -> CONFIG_DMA_CMA will be removed from .config
>>
>> Note: drivers/remoteproc seems to be special; commit c51e882cd711
>> ("remoteproc/davinci: Update Kconfig to depend on DMA_CMA") explains that
>> there is a real dependency to DMA_CMA for it to work; leave that dependency
>> in place and don't convert it to a soft dependency.
> 
> I don't think this dependency is fundamentally different from the others,
> though davinci machines tend to have less memory than a lot of the
> other machines, so it's more likely to fail without CMA.
> 

I was also unsure - and Lucas had similar thoughts. If you want, I can 
send a v4 also taking care of this.

Thanks!

> Regardless of this,
> 
> Reviewed-by: Arnd Bergmann <arnd@arndb.de>
> 


-- 
Thanks,

David / dhildenb