linux-kernel - Re: [PATCH] mmc: dw_mmc: use resource_size

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Date:	Wed, 18 Nov 2015 16:45:23 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	Andy Shevchenko <andy.shevchenko@...il.com>
Cc:	Ulf Hansson <ulf.hansson@...aro.org>,
	Jaehoon Chung <jh80.chung@...sung.com>,
	"linux-mmc@...r.kernel.org" <linux-mmc@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-arm Mailing List <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] mmc: dw_mmc: use resource_size_t to store physical address

On Wednesday 18 November 2015 17:29:19 Andy Shevchenko wrote:
> 
> I understand most of the things here, what I don't is how a platform
> is supposed to work if you have the following:
> a) HW, that uses register space let's say higher than 32-bit;
> b) DMA engine, which should provide a DMA capability for above HW block;
> c) dma_addr_t which does not cover the HW register space.

On this platform, the current code is obviously broken, because the pointer
is 32-bit wide and cannot reach the registers. I assume you agree on that
part.

With my patch, the 64-bit resource_size_t in dw_mci helps get the
correct FIFO address to this line:

	cfg.dst_addr = host->phy_regs + fifo_offset;

There, it remains broken because of the dma_addr_t being too short, and
we also need Linus' patch from https://lkml.org/lkml/2013/4/26/120 in
addition to mine.

> For me it clearly looks like a platform (HW / SW) configuration issue.

I think some people have argued in the past that we should always use
the same type for dma_addr_t, resource_size_t and phys_addr_t. That
would certainly fix the problem you describe as well. In practice,
everyone has that already, and my patch by itself fixes all the
cases where the FIFO is at a high address and dma_addr_t is already
64-bit wide.

> In case of bounce buffers I can't understand how it helps there.

Right, bounce buffers are irrelevant here, because the FIFO address
is never translated and never bounced.

	Arnd
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