[PATCH 1/5] spi/mxs: Fix extra CS pulses and read mode in multi-transfer messages

[Marek Vasut]

Dear Trent Piepho,

> On Mon, Apr 1, 2013 at 9:22 PM, Marek Vasut <marex at denx.de> wrote:
> >> On Mon, Apr 1, 2013 at 4:11 PM, Marek Vasut <marex at denx.de> wrote:
> >> >> +#define TXRX_WRITE           1       /* This is a write */
> >> >> +#define TXRX_DEASSERT_CS     2       /* De-assert CS at end of txrx
> >> >> */
> > 
> > btw. is it necessary to define new flags or is it possible to reuse some?
> 
> I don't see any others that would make sense.  The driver doesn't
> define any flags at all.  The spi layer doesn't have ones that match.
> And this is for a private static interface inside the driver.  It
> wouldn't be right to co-opt flags defined for some other purpose.

Ok, makes sense then.

btw. if defining bitfield flags, it makes more sense to use (1 << n) notation to 
make it more readable and also less prone to people using the actual flag value 
as a bitshift argument.

> >> >>       ctrl0 = readl(ssp->base + HW_SSP_CTRL0);
> >> >> 
> >> >> -     ctrl0 &= ~BM_SSP_CTRL0_XFER_COUNT;
> >> >> +     ctrl0 &= ~BM_SSP_CTRL0_XFER_COUNT & ~BM_SSP_CTRL0_IGNORE_CRC &
> >> >> +              ~BM_SSP_CTRL0_READ;
> >> > 
> >> > Well ... ctrl0 &= ~(FOO | BAR | BAZ); ... won't cut it?
> >> 
> >> Well, by De Morgan's law the two expressions are the same.  And they
> >> are the same number of characters.  And both will produce a compile
> >> time constant and thus the same code.  However, I like the ~FOO & ~BAR
> >> & ~BAR form better as there is greater parallelism between each term
> >> of the expression.
> > 
> > What about the law of readability of code ? ;-)
> 
> Don't see anything in CodingStyle that one should be preferred over
> the other.

There ain't any I'm aware of, but to paraphrase you, let's keep the format 
that's already used in the driver ;-) :

114         if (dev->mode & ~(SPI_CPOL | SPI_CPHA))

> I think this way is more readable than the other.
> Generally you can think of masking off bits via bitwise-and and
> setting bits with bitwise-or.  So if you want to do a sequence of
> masks, then using a sequence of bitwise-ands is the most readable and
> straightforward code IMHO.  Combing bits to mask with bitwise-or, then
> inverting the set and masking with that seems like a less clear way of
> doing the same thing.  And this way the existing tokens aren't
> modified, so there is less churn!
> 
> >> The DMA does write
> >> ctrl0 on each segment of each transfer, but look at the previous
> >> paragraph to see where it comes from.  Once READ or IGNORE_CRC are
> >> set, nothing will clear them until PIO mode is used.
> > 
> > When using a flash, the DMA and PIO mode usage is intermixed. Usually,
> > the PIO is used to program the command and then DMA to transmit the
> > payload. It's hardly ever PIO only orr DMA only for the whole transfer.
> 
> Probably why the problem hasn't been noticed.  It's obvious from a
> cursory examination of the driver that bits in ctrl0 are only SET in
> the DMA code, never cleared.  The PIO code does clear them.
> 
> >> It's the same
> >> with the bug fix for 3.9 that clears the XFER_COUNT bits in DMA mode.
> >> Strangely, I didn't notice that bug since all my xfers are the same
> >> length!  But I do switch between slaves, between read and write, and
> >> use messages with multiple transfers, in DMA mode, all of which are
> >> broken in the current driver.
> > 
> > btw. are you using MX23 or MX28 to test this?
> 
> IMX28.

Is that any mainline board?

> >> Existing code that uses the first/last flags is wrong and needs to be
> >> changed.  Therefor code using 'first' and 'last' will be changed.
> >> 
> >> Passing the flags as pointers is bad practice and makes no sense to
> >> do.  Does it make any sense to re-write code fix the way it uses first
> >> and last, then re-write those same lines a second time to not use
> >> pointers?
> > 
> > You can always flip it the other way -- first rework the use of flags,
> > then apply the fix. It'd make the patch much easier to understand, don't
> > you think?
> 
> So I should change 'first' to not be a pointer to an integer in one
> patch, then delete the flag entirely in another patch?

I'd say make a patch (0001) that implements the transition to using your newly 
defined flags and then make a patch (0002) that "Fix extra CS pulses and read 
mode in multi-transfer messages". One patch shall do one thing, that's the usual 
rule of the thumb. Obviously keep them in a series if these patches shall go in 
together. And why doesn't squashing them all together work you might ask -- 
because reviewing the result is hard.

[...]

Best regards,
Marek Vasut