Re: Problem mapping small PCI memory space.

["Gianluca Varenni" <gianluca.varenni@gmail.com>]

----- Original Message ----- 
From: "Richard B. Johnson" <linux-os@analogic.com>
To: "Gianluca Varenni" <gianluca.varenni@gmail.com>
Cc: "Linux kernel" <linux-kernel@vger.kernel.org>
Sent: Thursday, May 19, 2005 12:43 PM
Subject: Re: Problem mapping small PCI memory space.


> On Thu, 19 May 2005, Gianluca Varenni wrote:
>
>> Hi all.
>>
>> I'm writing a driver for a PCI board that exposes two memory spaces (out 
>> of
>> the 6 IO address regions).
>>
>> One of them is 1MB, and I can map it to user level without problems. The
>> other one is only 512 bytes.
>> If I try to open it with /dev/mem, it returns EINVAL (the 1MB memory 
>> space
>> is opened without any problem). If I try to expose it through mmap, mmap
>> succeeds, but I only see garbage at user level. At kernel level, I can
>> access that 512 bytes memory by using ioremap() on the physical address
>> returned by pci_resource_start().
>>
>> Are there any lower limits on the size of a PCI memory region?
>>
>> Have a nice day
>> GV
>>
>
> You impliment mmap() in your driver. It accesses the first megabyte
> as 256 pages. Then you tack on the additional page that your 512
> bytes resides at. mmap() only works with pages. The pages must
> be ioremap_nocache and they must be reserved. The reserved part
> is important to have them visible from user-space using mmap.
>
> That's IFF you really need to see the stuff in user-mode. Normally,
> you write a driver that accesses everything using the PCI primatives
> provided in the kernel, for the kernel.

Do you have any link to a linux driver working exactly like this?

Have a nice day
GV

>
>
> Cheers,
> Dick Johnson
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