[PATCH v2 0/3] fix memremap on ARM

[Dan Williams]

On Thu, Mar 3, 2016 at 9:41 AM, Ard Biesheuvel
<ard.biesheuvel at linaro.org> wrote:
> On 3 March 2016 at 18:33, Dan Williams <dan.j.williams at intel.com> wrote:
>> On Thu, Mar 3, 2016 at 9:30 AM, Ard Biesheuvel
>> <ard.biesheuvel at linaro.org> wrote:
>>> On 3 March 2016 at 18:27, Dan Williams <dan.j.williams at intel.com> wrote:
>>>> On Thu, Mar 3, 2016 at 9:16 AM, Ard Biesheuvel
>>>> <ard.biesheuvel at linaro.org> wrote:
>>>>> On 3 March 2016 at 18:13, Russell King - ARM Linux
>>>>> <linux at arm.linux.org.uk> wrote:
>>>>>> On Thu, Mar 03, 2016 at 05:24:41PM +0100, Ard Biesheuvel wrote:
>>>>>>> If there are no remaining objections, may I suggest that Dan takes
>>>>>>> patch #1 and #2, and that I put the third patch into Russell's patch
>>>>>>> system? The only strict ordering requirement is that #1 is merged
>>>>>>> before #2 and #3 both hit mainline, and that is solved by taking #1
>>>>>>> and #2 in order.
>>>>>>
>>>>>> How does the dependency between 1 and 3 get satisfied?  Your comment
>>>>>> makes no sense to me.
>>>>>>
>>>>>
>>>>> Patch #3 introduces a function arch_memremap_wb() on the ARM side
>>>>> which will never get called unless patch #2 is also merged. Once both
>>>>> #2 and #3 are in, the memremap() call that #1 reverts will use
>>>>> MT_MEMORY_RW attributes rather than MT_DEVICE_CACHED, so #1 needs to
>>>>> go first, but #2 and #3 can go in in either order.
>>>>>
>>>>> In other words, there are no build dependencies between the patches,
>>>>> but to prevent pxa2xx-flash from using MT_MEMORY_RW attributes, it
>>>>> needs to go in before #2
>>>>
>>>> Can we add a new MEMREMAP_ type for this case so that we don't need to
>>>> keep carrying the confusing ioremap_cache() which means different
>>>> things to different archs and silently falls back to uncached on some
>>>> archs.
>>>
>>> Actually, I don't think so. Unifying ioremap_cache() between
>>> architectures was a mistake to begin with, since I/O mapping
>>> attributes vary so wildly between architectures, and there are very
>>> few drivers that depend on such specific behavior that actually need
>>> to be built for multiple architectures.
>>>
>>> So I think your memremap() is a fantastic idea, considering the
>>> widespread abuse of ioremap_cache(), also in common code. But removing
>>> it altogether to replace it with something that looks generic but
>>> never is, is just repeating the same mistake imo.
>>
>> I'm not suggesting it be a generic flag.  I.e. something like
>> MEMREMAP_ARM_ that explicitly documents that this driver has arch
>> specific mapping dependencies.  That request type will fail on any
>> arch that does not implement MEMREMAP_ARM_ support.
>
> I don't see the point of adding this to a generic API. ioremap_cache()
> is used to perform I/O, even if it has memory semantics on read.
> Getting rid of the abuse, where ioremap_cache() is used to map system
> RAM to access ACPI tables etc is something that we should fix, yes,
> but arch specific I/O needs to stay out of the picture imo, simply
> because there is no problem to fix here.

Hmm, then what about Russell's suggestoin, if I'm not
mischaracterizing, that ioremap_cache() move to its old
ioremap_cached() name.  The latter has less cross-arch confusion.