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

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On 2021-04-09 14:39, David Hildenbrand wrote:
On 09.04.21 15:35, Arnd Bergmann wrote:
On Fri, Apr 9, 2021 at 1:21 PM David Hildenbrand <david@xxxxxxxxxx> 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.

TBH I think it should all just be removed. DMA_CMA is effectively an internal feature of the DMA API, and drivers which simply use the DMA API shouldn't really be trying to assume *how* things might be allocated at runtime - CMA is hardly the only way. Platform-level assumptions about the presence or not of IOMMUs, memory carveouts, etc., and whether it even matters - e.g. a device with a tiny LCD may only need display buffers which still fit in a regular MAX_ORDER allocation - could go in platform-specific configs, but I really don't think they belong at the generic subsystem level.

We already have various examples like I2S drivers that won't even probe without a dmaengine provider being present, or host controller drivers which are useless without their corresponding phy driver (and I'm guessing you can probably also do higher-level things like include the block layer but omit all filesystem drivers). I don't believe it's Kconfig's job to try to guess whether a given configuration is *useful*, only to enforce that's it's valid to build.

Robin.



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