On 08/19/2016 04:59 PM, Andrew Lunn wrote: > On Fri, Aug 19, 2016 at 11:16:14AM +0200, Zefir Kurtisi wrote: >> The eTSEC register MRBLR defines the maximum space in >> the RX buffers and is set to 1536 by gianfar. This >> reasonably covers the common use case where the MTU >> is kept at default 1500. >> >> Alas, if the eTSEC is attached to a DSA enabled switch, >> the DSA header extension causes every maximum sized >> frame to be fragmented by the hardware (1536 + 4). >> >> This patch increases the maximum RX buffer size by >> RBUF_ALIGNMENT (64) octets. Since the driver uses a >> half-page memory schema, this change does not >> increase allocated memory but allows the hardware to >> use 1600 bytes of the totally available 2048. >> >> Signed-off-by: Zefir Kurtisi <zefir.kurt...@neratec.com> >> --- >> drivers/net/ethernet/freescale/gianfar.h | 3 ++- >> 1 file changed, 2 insertions(+), 1 deletion(-) >> >> diff --git a/drivers/net/ethernet/freescale/gianfar.h >> b/drivers/net/ethernet/freescale/gianfar.h >> index 373fd09..02b794b 100644 >> --- a/drivers/net/ethernet/freescale/gianfar.h >> +++ b/drivers/net/ethernet/freescale/gianfar.h >> @@ -100,7 +100,8 @@ extern const char gfar_driver_version[]; >> #define DEFAULT_RX_LFC_THR 16 >> #define DEFAULT_LFC_PTVVAL 4 >> >> -#define GFAR_RXB_SIZE 1536 >> +/* prevent fragmenation by HW in DSA environments */ >> +#define GFAR_RXB_SIZE (1536 + RXBUF_ALIGNMENT) > > Hi Zefir > > Using RXBUF_ALIGNMENT suggests this has something to do with > alignment, not extra headers. > > How about > > /* Prevent fragmenation by HW when using extra headers like DSA */ > #define GFAR_RXB_SIZE (1536 + 8) > > Andrew > Hi Andrew,
the MRBLR has to be written with multiples of 64 (lower 6 bits reserved as 0), therefore (1536 + 64) is the next valid value to chose. As noted in the commit message, this is an artificial limitation, since the RX buffers are essentially 2048 bytes (GFAR_RXB_TRUESIZE) long. I don't fully understand why GFAR_RXB_SIZE has to be lower than that, preventing the HW using all available memory - the freescale guys most probably do. Cheers, Zefir
