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    SubjectRe: [PATCH v2 1/2] dmaengine: stm32-mdma: align TLEN and buffer length on burst
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    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.mordret@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.

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