On Tue, 23 Sep 2008 08:23:25 +0100, Russell King - ARM Linux wrote: > On Tue, Sep 23, 2008 at 09:31:09AM +0800, Eric Miao wrote: > > > Allocating such small buffers using kmalloc seems pretty inefficient. > > > At the very least, I would allocate both buffers at once. But quite > > > frankly I don't get why you don't just make these buffers part of > > > struct max1111_data. This would even make the structure smaller! > > > > > > > I originally place the buffer within "struct max1111_data" but received > > a mail from David Brownell suggesting using a kmalloc() buffer, so that > > DMA mode will work better with the cache alignment and trailing bytes, > > though PIO can just work happily. I don't know the specific reason for > > this, honestly. > > Having cachelines overlap with other data which may be modified during > the DMA causes problems on non-cache coherent hardware. It's much > safer to ensure that DMA buffers don't share cache lines with anything > else. Ah, OK. I had not realized that these buffers were used for DMA, sorry. Still, can't we allocate the transmit and receive buffers in one go? I guess transmit and receive can't happen concurrently, so that would be safe? -- Jean Delvare
