From mboxrd@z Thu Jan 1 00:00:00 1970 From: Troy Kisky Date: Fri, 10 May 2013 13:54:36 -0700 Subject: [U-Boot] [PATCH 2/2] spi: mxc_spi: Update pre and post divider algorithm In-Reply-To: <518D45BB.80308@gmail.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> <518D45BB.80308@gmail.com> Message-ID: <518D5E8C.7090003@boundarydevices.com> List-Id: MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: u-boot@lists.denx.de On 5/10/2013 12:08 PM, Dirk Behme wrote: > 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. __udivsi3 already has this optimization. fls can be 2 instructions clz r0,r0 rsb r0,r0,#32 so your savings are a subroutine call and a return in the unlikely case that (clk_src <= max_hz) But I don't have a real objection to leaving it, so feel free to decide. > >> 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 >> >> > >