From mboxrd@z Thu Jan 1 00:00:00 1970 From: Dirk Behme Date: Fri, 10 May 2013 21:08:43 +0200 Subject: [U-Boot] [PATCH 2/2] spi: mxc_spi: Update pre and post divider algorithm In-Reply-To: <518D3FF9.1030605@boundarydevices.com> References: <1368076793-24438-1-git-send-email-dirk.behme@gmail.com> <1368076793-24438-2-git-send-email-dirk.behme@gmail.com> <518BE446.9070306@boundarydevices.com> <518C86E2.4050505@gmail.com> <518D3FF9.1030605@boundarydevices.com> Message-ID: <518D45BB.80308@gmail.com> List-Id: MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: u-boot@lists.denx.de Am 10.05.2013 20:44, schrieb Troy Kisky: > On 5/9/2013 10:34 PM, Dirk Behme wrote: >> Am 09.05.2013 20:00, schrieb Troy Kisky: >>> On 5/8/2013 10:19 PM, Dirk Behme wrote: >>>> The spi clock divisor is of the form x * (2**y), or x << y, where >>>> x is >>>> 1 to 16, and y is 0 to 15. Note the similarity with floating point >>>> numbers. >>>> Convert the desired divisor to the smallest number which is >= >>>> desired divisor, >>>> and can be represented in this form. The previous algorithm chose a >>>> divisor >>>> which could be almost twice as large as needed. >>>> >>>> Signed-off-by: Troy Kisky >>>> Signed-off-by: Dirk Behme >>>> --- >>>> drivers/spi/mxc_spi.c | 27 ++++++++++++--------------- >>>> 1 file changed, 12 insertions(+), 15 deletions(-) >>>> >>>> diff --git a/drivers/spi/mxc_spi.c b/drivers/spi/mxc_spi.c >>>> index 3e903b3..66c2ad8 100644 >>>> --- a/drivers/spi/mxc_spi.c >>>> +++ b/drivers/spi/mxc_spi.c >>>> @@ -128,7 +128,7 @@ static s32 spi_cfg_mxc(struct mxc_spi_slave >>>> *mxcs, unsigned int cs, >>>> unsigned int max_hz, unsigned int mode) >>>> { >>>> u32 clk_src = mxc_get_clock(MXC_CSPI_CLK); >>>> - s32 pre_div = 1, post_div = 0, i, reg_ctrl, reg_config; >>>> + s32 pre_div = 1, post_div = 0, reg_ctrl, reg_config; >>> >>> First, I'm totally fine with the patch as it is. I'm just going to >>> point out things you may want to change, or >>> send a follow-up patch. >>> >>> Here, no need to initialize pre_div, post_div, if you delete the if >>> (clk_src > max_hz) below which is not needed. >> >> Hmm, why is it not needed? >> >> If you remove the if, you *always* do at least the computation >> >> pre_div = DIV_ROUND_UP(clk_src, max_hz); >> post_div = fls(pre_div - 1); >> if (post_div > 4) { >> >> I would think that doing the initialization and the if is much >> cheaper than always doing above computation, even if its not needed? >> I would keep the if. >> >>>> u32 ss_pol = 0, sclkpol = 0, sclkpha = 0; >>>> struct cspi_regs *regs = (struct cspi_regs *)mxcs->base; >>>> @@ -147,27 +147,24 @@ static s32 spi_cfg_mxc(struct mxc_spi_slave >>>> *mxcs, unsigned int cs, >>>> reg_ctrl |= MXC_CSPICTRL_EN; >>>> reg_write(®s->ctrl, reg_ctrl); >>>> - /* >>>> - * The following computation is taken directly from Freescale's >>>> code. >>>> - */ >>>> if (clk_src > max_hz) { >>> This "if" can be removed. >>> >>>> pre_div = DIV_ROUND_UP(clk_src, max_hz); >>> If you subtract -1 here instead of when you set the divisor register, >>> the logic becomes simpler >>> >>> >>> pre_div = DIV_ROUND_UP(clk_src, max_hz) - 1; >>> >>> or just >>> >>> pre_div = (clk_src - 1) / max_hz; >>> >>>> - if (pre_div > 16) { >>>> - post_div = pre_div / 16; >>>> - pre_div = 16; >>>> - } >>>> - if (post_div != 0) { >>>> - for (i = 0; i < 16; i++) { >>>> - if ((1 << i) >= post_div) >>>> - break; >>>> - } >>>> - if (i == 16) { >>>> + /* fls(1) = 1, fls(0x80000000) = 32, fls(16) = 5 */ >>>> + post_div = fls(pre_div - 1); >>>> + if (post_div > 4) { >>>> + post_div -= 4; >>>> + >>>> + if (post_div >= 16) { >>>> printf("Error: no divider for the freq: %d\n", >>>> max_hz); >>>> return -1; >>>> } >>>> - post_div = i; >>>> + pre_div = (pre_div + (1 << post_div) - 1) >> post_div; >>> >>> This would become >>> pre_div >>= post_div; >>>> + >>>> + } else { >>>> + post_div = 0; >>>> } >>>> + >>>> } >>>> debug("pre_div = %d, post_div=%d\n", pre_div, post_div); >>> >>> And >>> >>> MXC_CSPICTRL_PREDIV(pre_div - 1); >>> >>> would return to >>> >>> MXC_CSPICTRL_PREDIV(pre_div); >> >> Well, where to do the -1 mainly depends on how we want to interpret >> the output of >> >> debug("pre_div = %d, post_div=%d\n", pre_div, post_div); > I'd change to > > debug("actual div = %d, pre_div = %d, post_div=%d\n", (pre_div + 1) << > post_div, pre_div, post_div); > > to eliminate confusion. > > >> >> There are two options how to understand this, at least the pre_div = >> %d part: >> >> a) print the pre_div as the real divider used in a human readable >> format. I.e. if we divide by /15 then print 15 >> >> or >> >> b) print the pre_div value we write to the register. I.e. if we >> divide by /15 then print 14 >> >> Up to now we are doing (a) and calculate the register value after >> the debug print. Your proposal would switch this to (b). Anyway, if >> it makes the algorithm simpler I'd agree that it's fine to switch to >> (b). >> >> To summarize, this would become then: >> >> s32 pre_div = 1, post_div = 0, reg_ctrl, reg_config; >> >> ... > > If you keep the "if", pre_div = 0 Ah, yes, thanks. > Also, I'd change s32 to unsigned or u32. > > But usually the if will be true, so why keep it? For cases where clk_src <= max_hz. Then don't do the calculation because it's unnecessary. > It is just code bloat and prone to errors like yours above. > > The only logic difference between keeping/killing the "if" is when > clk_src == 0. Keeping will give a divide by 1. Killing will give an > error. > I'd argue that an error is more appropriate. > >> >> if (clk_src > max_hz) { >> pre_div = (clk_src - 1) / max_hz; >> /* fls(1) = 1, fls(0x80000000) = 32, fls(16) = 5 */ >> post_div = fls(pre_div); >> if (post_div > 4) { >> post_div -= 4; >> if (post_div >= 16) { >> printf("Error: no divider for the freq: %d\n", max_hz); >> return -1; >> } >> pre_div >>= post_div; >> } else { >> post_div = 0; >> } >> } >> >> ... >> >> MXC_CSPICTRL_PREDIV(pre_div); >> >> Is this correct? >> >> Best regards >> >> Dirk > >