linux-kernel - Re: [PATCH] USB: f_mass_storage: dynamic buffers for better alignment

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Message-ID: <20100315192812.GE25452@gandalf>
Date:	Mon, 15 Mar 2010 21:28:13 +0200
From:	Felipe Balbi <me@...ipebalbi.com>
To:	Micha?? Nazarewicz <m.nazarewicz@...sung.com>
Cc:	me@...ipebalbi.com, linux-usb@...r.kernel.org,
	David Brownell <dbrownell@...rs.sourceforge.net>,
	gregkh@...e.de, linux-kernel@...r.kernel.org,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Kyungmin Park <kyungmin.park@...sung.com>
Subject: Re: [PATCH] USB: f_mass_storage: dynamic buffers for better
 alignment

On Mon, Mar 15, 2010 at 08:20:08PM +0100, Micha?? Nazarewicz wrote:
> > On Mon, Mar 15, 2010 at 11:09:55AM +0100, Michal Nazarewicz wrote:
> >> "Static" buffers in fsg_buffhd structure (ie. fields which are arrays
> >> rather then pointers to dynamically allocated memory) are not aligned
> >> to any "big" power of two which may lead to poor DMA performance
> 
> On Mon, 15 Mar 2010 19:10:21 +0100, Felipe Balbi <me@...ipebalbi.com> wrote:
> > not so true as you can add __attribute__ ((aligned(32))) to those.
> 
> I admit, I haven't thought about that.  Some fields rearrangement
> could help avoid some padding but yes, it can be done.
> 
> However, there is one more thing I've had in mind.  Each buffer
> is 4 pages (16 KiB) and there are two such buffers in struct
> fsg_common therefore the whole size of the structure is
> 9 pages (> 32 KiB).
> 
> I've been simply concerned about using kamlloc() for such big
> structures so in the end decided to split it into 3 allocations.
> 
> Maybe I'm overeating though?  Or maybe vmalloc() would solve those
> problems?  But then again, vmalloc() could degrade DMA performance
> on systems w/o scatter-gather.
> 
> What do you think?

I have no opinion anymore :-p

I can only think about the devices I've been working on which would be a
pain to allocate so much memory and would suffer if you use vmalloc()
too, so both would be a no-no for me :-p

> 	bh = common->buffhds;
> 	rc = -ENOMEM;
> 	i = FSG_NUM_BUFFERS;
> 	for(;;) {
> 		bh->buf = kmalloc(FSG_BUFLEN, GFP_KERNEL);
> 		if (unlikely(!bh->buf))
> 			goto error_release;
> 		if (!--i)
> 			break;
> 		bh->next = bh + 1;
> 		++bh;
> 	}
> 	bh->next = common->buffhds;
> 
> What do you think?

how about ?

for (i = FSG_NUM_BUFFER; i; i--, ++bh) {
	bh->buf = kmalloc(FSG_BUFLEN, GFP_KERNEL);
	if (!bh->buf)
		goto error_release;
}

-- 
balbi
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