Re: How to map high memory for block io

[Pierre Ossman <drzeus-list@drzeus.cx>]

Jens Axboe wrote:
> On Fri, Jan 27 2006, Russell King wrote:
>   
>> On Fri, Jan 27, 2006 at 10:58:59PM +0100, Pierre Ossman wrote:
>>     
>>> Test done here, few minutes ago. Added this to the wbsd driver in its
>>> kmap routine:
>>>
>>>     if ((host->cur_sg->offset + host->cur_sg->length) > PAGE_SIZE)
>>>         printk(KERN_DEBUG "wbsd: Big sg: %d, %d\n",
>>>             host->cur_sg->offset, host->cur_sg->length);
>>>
>>> got:
>>>
>>> [17385.425389] wbsd: Big sg: 0, 8192
>>> [17385.436849] wbsd: Big sg: 0, 7168
>>> [17385.436859] wbsd: Big sg: 0, 7168
>>> [17385.454029] wbsd: Big sg: 2560, 5632
>>> [17385.454216] wbsd: Big sg: 2560, 5632
>>>       
>> Jens - what's going on?  These look like invalid sg entries to me.
>>
>> If they are supposed to be like that, there will be additional problems
>> for block drivers ensuring cache coherency on PIO.
>>     
>
> No freaking idea, must be coming out of the pci dma mapping. The IOMMU
> doing funky stuff? How are these sg lists mapped?
>
>   

This is an ISA (i.e. platform) device, so no PCI involved. There is also
no IOMMU on this system.

As for the mapping there doesn't seem to be anything fancy about it
(this is Russell's territory so this is just my naive view of it). The
queue is set up in mmc_queue.c and the sg is mapped using
blk_rq_map_sg() in mmc_block.c.

But if sg entries are not supposed to cross pages, then I guess that
means that any transfer is limited in size by PAGE_SIZE *
min(max_phys_seg, max_hw_seg), right?

Rgds
Pierre