linux-kernel - Re: [20/80] ALSA: lx6464es - fix device communication via command bus

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Date:	Fri, 09 Dec 2011 15:17:04 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Greg KH <gregkh@...e.de>, Ingo Molnar <mingo@...e.hu>,
	Brian Gerst <brgerst@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>, x86@...nel.org,
	linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	akpm@...ux-foundation.org, alan@...rguk.ukuu.org.uk,
	Tim Blechmann <tim@...ngt.org>, Takashi Iwai <tiwai@...e.de>
Subject: Re: [20/80] ALSA: lx6464es - fix device communication via command
 bus

Sorry for the late reply.  With baby prep and everything going on I'm a
bit ADD.

On 12/07/2011 10:09 AM, Linus Torvalds wrote:
> On Wed, Dec 7, 2011 at 10:00 AM, H. Peter Anvin <hpa@...or.com> wrote:
>>
>> I agree in principle, although I am really not happy about the idea of
>> disallowing 64-bit writes from device drivers that haven't explicitly
>> indicated they won't support it.
> 
> I'd really prefer going in the other direction. Especially since
> 64-bit writes would be optional *anyway* (ie impossible on 32-bit
> hosts). So a driver should explicitly say "I can do 64-bit stuff"
> rather than having to say "I cannot do it".
> 
> And if a driver *has* to have 64-bit accesses, it had better do them
> all by hand, using "writeq()" and being dependent on the architecture
> actually having that support. No "memcpy_toio" and friends at all.
> 
> So adding a "memcpy_toio64()" that defaults to the normal 32-bit max
> but *might* do 64-bit copies sounds fine to me. But we really should
> default to something that is consistent across architectures, and that
> is not 64-bit accesses.

I feel a bit funny about memcpy_toio64() being able to *not* to 64-bit
copies... it feels like it really violates the assumptions.

What I would suggest, technically is:

- memcpy_toio() becomes a legacy alias for memcpy_toio32().
- memcpy_toio32() and memcpy_toio64() are defined to do exactly
  those reference sizes.  If the platform can't do 64-bit I/O
  then memcpy_toio64 is undefined (like readq/writeq).
- we introduce memcpy_toio_fast() which is explicitly allowed to do any
  kind of references, including larger than 64 bits if supported in
  future CPUs.

However, I *also* found that we have a metric boatload of partially
overlapping redundant aliases:

iowrite{8,16,32}_rep()
	[but iowrite64_rep() is missing]

__iowrite{32,64}_copy()

memcpy_toio()

What I have proposed to memcpy_toio{32,64}() pretty much matches
__iowrite_{32,64}_copy() and those functions should probably be
renamed... not sure what the sane thing to do with the iowrite functions
is here.

	-hpa
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