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

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Message-id: <op.u9mjqvz87p4s8u@pikus>
Date:	Mon, 15 Mar 2010 20:43:33 +0100
From:	Michał Nazarewicz <m.nazarewicz@...sung.com>
To:	me@...ipebalbi.com
Cc:	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:
>> Each buffer is 4 pages and there are two such buffers in struct
>> fsg_common therefore the size of the structure is 9 pages.
>>
>> 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.

On Mon, 15 Mar 2010 20:28:13 +0100, Felipe Balbi <me@...ipebalbi.com> wrote:
> 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

So here it is...  I think allocating buffers dynamically via kmalloc()
is the safest way. :)

>> 	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;

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

bh->next cyclic buffer needs to be initialised.  Otherwise I'd stick
with what you've posted.

-- 
Best regards,                                           _     _
  .o. | Liege of Serenely Enlightened Majesty of       o' \,=./ `o
  ..o | Computer Science,  Michał "mina86" Nazarewicz     (o o)
  ooo +---[mina86@...a86.com]---[mina86@...ber.org]---ooO--(_)--Ooo--
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