linux-kernel - Re: Linux UIO driver cache problem in PowerPC (fix)

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Date:	Thu, 13 Mar 2008 08:19:32 +0100
From:	Hans-Jürgen Koch <hjk@...utronix.de>
To:	"Jean-Samuel Chenard" <jsamch@...il.com>
Cc:	"Thomas Gleixner" <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>, Greg KH <greg@...ah.com>,
	grant.likely@...retlab.ca
Subject: Re: Linux UIO driver cache problem in PowerPC (fix)

Am Wed, 12 Mar 2008 15:22:59 -0400
schrieb "Jean-Samuel Chenard" <jsamch@...il.com>:

> Hi,

Hi Jean-Samuel,
thanks for your report. Please CC LKML if you find bugs like this. I
also added Grant Likely (PPC Xilinx Virtex maintainer), as it seems to
be a platform specific problem.

> 
> Experimenting with your userspace I/O driver on a Xilinx FPGA (PowerPC
> 405), I was having problems when reading hardware device registers
> mapped on my platform using the user-space mmap() call.
> 
> After some investigation (looking at the driver/char/mspec.c file), I
> found that preventing caching of the page in uio_mmap_physical fixed
> my issues with userspace mmap() reading messed-up values.

Hm, we already mark the page with VM_IO. That's sufficient on x86 and
arm. I'm not sure whether this is a bug in PPC memory handling. Why do
they cache VM_IO pages? 

> 
> This patch is against your initial commit of the UIO driver in the
> mainline kernel tree.
> 
> ====
> diff --git a/drivers/uio/uio.c b/drivers/uio/uio.c
> index 865f32b..36e1123 100644
> --- a/drivers/uio/uio.c
> +++ b/drivers/uio/uio.c
> @@ -22,6 +22,7 @@
>  #include <linux/string.h>
>  #include <linux/kobject.h>
>  #include <linux/uio_driver.h>
> +#include <asm/pgtable.h>
> 
>  #define UIO_MAX_DEVICES 255
> 
> @@ -447,6 +448,8 @@ static int uio_mmap_physical(struct
> vm_area_struct *vma)
> 
>         vma->vm_flags |= VM_IO | VM_RESERVED;
> 
> +       vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
> +
>         return remap_pfn_range(vma,
>                                vma->vm_start,
>                                idev->info->mem[mi].addr >> PAGE_SHIFT,
> ====
> 
> I am a bit unsure if this will break something in any of your uses of
> the UIO driver (on other platforms than ppc), but it really fixes
> things for me.  

It should be OK, but unneccesary on other platforms. I have no
objections, but I fear in 6 months we'll see a patch removing that
line again because it's not needed... 

> Let me know if you need more details on what was
> happening on my platform before I added this statement.

I'd like to hear the opinion of people really involved in PPC memory
handling.

> 
> Thanks for the great driver for user-space I/O, it will save me lots
> of time in my research.
> 
> Regards,
> 
> Jean-Samuel

Thanks,
Hans

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