On 13/04/18 10:45, Pierre Yves MORDRET wrote:
Hi Robin

On 04/11/2018 05:14 PM, Robin Murphy wrote:
On 11/04/18 15:44, Pierre-Yves MORDRET wrote:
Both buffer Transfer Length (TLEN if any) and transfer size have to be
aligned on burst size (burst beats*bus width).

Signed-off-by: Pierre-Yves MORDRET <pierre-yves.mord...@st.com>
---
    Version history:
      v1:
         * Initial
      v2:
---
---
   drivers/dma/stm32-mdma.c | 2 +-
   1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/dma/stm32-mdma.c b/drivers/dma/stm32-mdma.c
index daa1602..fbcffa2 100644
--- a/drivers/dma/stm32-mdma.c
+++ b/drivers/dma/stm32-mdma.c
@@ -413,7 +413,7 @@ static u32 stm32_mdma_get_best_burst(u32 buf_len, u32 tlen, 
u32 max_burst,
        u32 best_burst = max_burst;
        u32 burst_len = best_burst * width;
- while ((burst_len > 0) && (tlen % burst_len)) {
+       while ((burst_len > 0) && (((tlen | buf_len) & (burst_len - 1)) != 0)) {
                best_burst = best_burst >> 1;
                burst_len = best_burst * width;
        }

FWIW, doesn't that whole loop come down to just:

        burst_len = min(ffs(tlen | buf_len), max_burst * width);

No sure it ends as expected. or I miss something or don't understand this 
statement
I tried with "relevant value" : i.e. best_burst = 32, Tlen=128(default) and
buf_len = 64, width= 4. This statements gets me something wrong output => 7
instead of 16 * 4.
I doubt :)

Heh, seems I confused myself halfway through and started thinking max_burst and width were the exponents x rather than the values 2^x...

A more representative guess should be:

        min(1 << __ffs(tlen | buf_len), max_burst * width);

but the general point I was trying to make is that a loop checking whether the bottom n bits of something are zero for different values of n is unnecessary when n can simply be calculated directly*.

Robin.


* in the case of this "just the lowest set bit" idiom there's also the shift-free ((x & (x - 1)) ^ x), but as well as being unreadable it's generally less efficient than (1 << __ffs(x)) for most modern ISAs.

Reply via email to