[PATCH/RFC] ps3/block: Add ps3vram-ng driver for accessing video RAM as block device

[Geert Uytterhoeven]

On Thu, 5 Mar 2009, Jens Axboe wrote:
> On Thu, Mar 05 2009, Geert Uytterhoeven wrote:
> > On Thu, 5 Mar 2009, Jens Axboe wrote:
> > > On Wed, Mar 04 2009, Geert Uytterhoeven wrote:
> > > > Below is the rewrite of the PS3 Video RAM Storage Driver as a plain block
> > > > device, as requested by Arnd Bergmann.

> > > I'd rewrite this as a ->make_request_fn handler instead. Then you can
> > > get rid of the kernel thread. IOW, change
> > >
> > > queue = blk_init_queue(ps3vram_request, &priv->lock);
> > >
> > > to
> > >
> > > queue = blk_alloc_queue(GFP_KERNEL);
> > > blk_queue_make_request(queue, ps3vram_make_request);
> > 
> > Thanks, I didn't know that part...
> > 
> > > Add error handling of course, and call blk_queue_max_*() to set your
> > > limits for this device.
> > 
> > I took out the blk_queue_max_*() calls (compared to ps3disk.c), as
> > none of the limits apply, and the defaults are fine.
> > 
> > Is that OK, or is it better to make it explicit?
> 
> I think it's always good to make it explicit. Plus for this case you
> definitely need it, as blk_init_queue() wont do it for you anymore.

blk_queue_make_request() does it for me, too:

void blk_queue_make_request(struct request_queue *q, make_request_fn *mfn)
{
	...
	blk_queue_max_phys_segments(q, MAX_PHYS_SEGMENTS);
	blk_queue_max_hw_segments(q, MAX_HW_SEGMENTS);
	...
	blk_queue_max_segment_size(q, MAX_SEGMENT_SIZE);
	...
	blk_queue_max_sectors(q, SAFE_MAX_SECTORS);
	...
}

struct request_queue *
blk_init_queue_node(request_fn_proc *rfn, spinlock_t *lock, int node_id)
{
	...
	blk_queue_max_segment_size(q, MAX_SEGMENT_SIZE);

	blk_queue_max_hw_segments(q, MAX_HW_SEGMENTS);
	blk_queue_max_phys_segments(q, MAX_PHYS_SEGMENTS);
	...
}

> > > Then add a ps3vram_make_request() ala:
> > 
> > > static void ps3vram_do_request(struct request_queue *q, struct bio *bio)
> > > {

> > > }
> > >
> > > I just typed it here, so if it doesn't compile you get to keep the
> > > pieces :-)
> > 
> > OK, I'll give it a try...
> > 
> > BTW, does this mean the `simple' way, which I used based on LDD3, is
> > deprecated?
> 
> Depends.. It's obviously not a very effective approach, since you punt
> to a thread for each request. But if you need the IO scheduler helping
> you with merging and sorting (for a rotational device), it still has
> some merit. For this particular case, the ->make_request_fn approach is
> much better.

Without the thread, performance indeed increased.

But then I noticed ps3vram_make_request() may be called concurrently, so I had
to add a mutex to avoid data corruption. This slows the driver down, and in the
end, the version with a thread turns out to be ca. 1% faster. The version
without a thread is about 50 lines less code, though.

With kind regards,

Geert Uytterhoeven
Software Architect

Sony Techsoft Centre Europe
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