linux-kernel - Re: [PATCH 2/2] New driver: Xillybus generic interface for FPGA (programmable logic)

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Date:	Fri, 30 Nov 2012 18:58:08 +0200
From:	Eli Billauer <eli.billauer@...il.com>
To:	Greg KH <gregkh@...uxfoundation.org>
Cc:	linux-kernel@...r.kernel.org, arnd@...db.de
Subject: Re: [PATCH 2/2] New driver: Xillybus generic interface for FPGA
 (programmable logic)

On 11/30/2012 06:32 PM, Greg KH wrote:
>
> As we need to review the user/kernel api here, putting the docs as part
> of the driver submission is a good idea :)
>
> I didn't know, nor do I trust, that a random web site would have the
> correct documentation for a kernel driver.
>    
OK. I'll add a file in Documentation/misc-devices/.
>>>> +#if (PAGE_SIZE<   4096)
>>>> +#error Your processor architecture has a page size smaller than 4096
>>>> +#endif
>>>>          
>>> That can never happen.  Even if it does, you don't care about that in
>>> the driver.
>>>
>>>        
>> I removed this check because it can't happen. But the driver *does*
>> care about this, since it creates a lot of buffers with different
>> alignments, hence depending on the pages' alignment.
>>      
> Alignment is different than the size of a page.  What happens if your
> driver runs on a machine with a page size bigger than 4K?  You need to
> be able to handle that properly, so perhaps you should check that?
>    
The problem is if the page size *smaller* than 4kB. The buffers 
allocated by the driver must not cross a 4kB boundary, and it's assumed 
that anything returned by __get_free_pages() is 4 kB-aligned. Otherwise 
the FPGA will generate illegal PCIe packets by crossing that boundary.

If the page boundary is bigger than 4k, the driver handles that well.

>
> It is no problem to create dozens of misc devices.  It makes your driver
> smaller, contain less code that I have to audit and you have to ensure
> you got right, and it removes another user of 'struct class' which we
> are trying to get rid of anyway.  So please, move to use a misc device.
>    
Very well. I'll remove that.

Thanks again for your comments. I'll prepare a v3.

    Eli

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