From: Michael Schmitz > Sent: 29 June 2022 00:09 > > Hi Arnd, > > On 29/06/22 09:50, Arnd Bergmann wrote: > > On Tue, Jun 28, 2022 at 11:03 PM Michael Schmitz <schmitzmic@xxxxxxxxx> wrote: > >> On 28/06/22 19:03, Geert Uytterhoeven wrote: > >>>> The driver allocates bounce buffers using kmalloc if it hits an > >>>> unaligned data buffer - can such buffers still even happen these days? > >>> No idea. > >> Hmmm - I think I'll stick a WARN_ONCE() in there so we know whether this > >> code path is still being used. > > kmalloc() guarantees alignment to the next power-of-two size or > > KMALLOC_MIN_ALIGN, whichever is bigger. On m68k this means it > > is cacheline aligned. > > And all SCSI buffers are allocated using kmalloc? No way at all for user > space to pass unaligned data? I didn't think kmalloc() gave any such guarantee about alignment. There are cache-line alignment requirements on systems with non-coherent dma, but otherwise the alignment can be much smaller. One of the allocators adds a header to each item, IIRC that can lead to 'unexpected' alignments - especially on m68k. dma_alloc_coherent() does align to next 'power of 2'. And sometimes you need (eg) 16k allocates that are 16k aligned. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
