About devm_platform_ioremap_resource [Was: Re: [PATCH 01/32] pwm: sun4i: convert to devm_platform_ioremap_resource]

[Robin Murphy]

On 2020-11-13 16:11, Thierry Reding wrote:
> On Fri, Nov 13, 2020 at 08:03:43AM +0100, Uwe Kleine-König wrote:
>> Hello,
>>
>> [Added lkml and the people involved in commit 7945f929f1a7
>> ("drivers: provide devm_platform_ioremap_resource()") to Cc:. For the
>> new readers: This is about patches making use of
>> devm_platform_ioremap_resource() instead of open coding it. Full context
>> at https://lore.kernel.org/r/20201112190649.GA908613@ulmo]
>>
>> On Thu, Nov 12, 2020 at 10:14:29PM +0100, Uwe Kleine-König wrote:
>>> On Thu, Nov 12, 2020 at 08:06:49PM +0100, Thierry Reding wrote:
>>>> I also think that it's overly narrow is scope, so you can't actually
>>>> "blindly" use this helper and I've seen quite a few cases where this was
>>>> unknowingly used for cases where it shouldn't have been used and then
>>>> broke things (because some drivers must not do the request_mem_region()
>>>> for example).
>>>
>>> You have a link to such an accident?
>>
>> I got a hint in private here: https://lore.kernel.org/r/1555670144-24220-1-git-send-email-aisheng.dong@nxp.com
>>
>> devm_platform_ioremap_resource() is platform_get_resource() +
>> devm_ioremap_resource() and here it was used to replace
>> platform_get_resource() + devm_ioremap().
>>
>> IMHO the unlucky thing in this situation is that devm_ioremap_resource()
>> and devm_ioremap() are different by more than just how they get the area
>> to remap. (i.e. devm_ioremap_resource() also does
>> devm_request_mem_region().)
>>
>> So the problem is not the added wrapper, but unclear semantics in the
>> functions it uses.
> 
> The semantics aren't unclear. It's just that the symbol name doesn't
> spell out every detail that the function implements, which, frankly, no
> function name ever does, at least not for anything beyond simple
> instructional examples. That's what we have documentation for and why
> people should read the documentation before they use a function and make
> (potentially wrong) assumption about what it does.
> 
>>                     In my eyes devm_ioremap() and
>> devm_platform_ioremap_resource() should better be named
>> devm_request_ioremap() and devm_platform_request_ioremap_resource()
>> respectively. Is it worth to rename these for clearity?
> 
> I think function names are always a compromise between giving you the
> gist of what the implementation does and being short enough so it
> doesn't become difficult to read or use.
> 
> One of the reasons why I dislike the addition of helpers for every
> common special case (like devm_platform_ioremap_resource()) is because
> it doesn't (always) actually make things easier for developers and/or
> maintainers. Replacing three lines of code with one is a minor
> improvement, even though there may be many callsites and therefore in
> the sum this being a fairly sizeable reduction. The flip side is that
> now we've got an extra symbol with an unwieldy name that people need
> to become familiar with, and then, like the link above shows, it doesn't
> work in all cases, so you either need to fall back to the open-coded
> version or you keep adding helpers until you've covered all cases. And
> then we end up with a bunch of helpers that you actually have to go and
> read the documentation for in order to find out which one exactly fits
> your use-case.
> 
> Without the helpers it's pretty simple to write, even if a little
> repetitive:
> 
>    1) get the resource you want to map
>    2) request the resource
>    3) map the resource
> 
> 2) & 3) are very commonly done together, so it makes sense to have a
> generic helper for them. If you look at the implementation, the
> devm_ioremap_request() implementation does quite a bit of things in
> addition to just requesting and remapping, and that's the reason why
> that helper makes sense.
> 
> For me personally, devm_platform_ioremap_resource() is just not adding
> enough value to justify its existence. And then we get all these other
> variants that operate on the resource name (_byname) and those which
> remap write-combined (_wc). But don't we also need a _byname_wc()
> variant for the combination? Where does it stop?

Arguably the worst thing about devm_platform_ioremap_resource() is that 
it was apparently the gateway drug to a belief that 
devm_platform_get_and_ioremap_resource() is anything other than a 
hideous way to obfuscate an assignment...

Robin.