Re: [PATCH] spi: tegra: add spi driver for SLINK controller

[Laxman Dewangan]

Thanks Stephen for review.
I have taken care of almost all feedback. Some of having my below comments.



On Tuesday 23 October 2012 01:32 AM, Stephen Warren wrote:
> On 10/18/2012 04:47 AM, Laxman Dewangan wrote:
> > Tegra20/Tegra30 supports the spi interface through its SLINK
> > controller. Add spi driver for SLINK controller.
>
> > +static inline void tegra_slink_writel(struct tegra_slink_data *tspi,
> > +             unsigned long val, unsigned long reg)
> > +{
> > +     writel(val, tspi->base + reg);
> > +
> > +     /* Read back register to make sure that register writes completed */
> > +     if (reg != SLINK_TX_FIFO)
> > +             readl(tspi->base + SLINK_MAS_DATA);
> Is that really necessary on every single write, or only for certain
> specific operations? This seems rather heavy-weight.

We do write register very less (as we  shadow the register locally), I 
do not see any perf degradation here.


> > +     val_write = SLINK_RDY;
> > +     val_write |= (val&  SLINK_FIFO_ERROR);
> Why not just always set SLINK_FIFO_ERROR; does it have to be set in the
> write only if the status was previously asserted? If that is true, how
> do you avoid a race condition where the bit gets set in SLINK_STATUS
> after you read it but before you write to clear it?
Status gets updated together. There is no steps of updating status.


> > +     if (bits_per_word == 8 || bits_per_word == 16) {
> > +             tspi->is_packed = 1;
> > +             tspi->words_per_32bit = 32/bits_per_word;
> Spaces required around all operators. Does this pass checkpatch? No:
> total: 1405 errors, 11 warnings, 1418 lines checked

I saw the issue by my checkpatch is not caching the issue. Let me 
recheck my command.

> > +             bits_per_word = t->bits_per_word ? t->bits_per_word :
> > +                                             tspi->cur_spi->bits_per_word;
> That logic is repeated a few times. Shouldn't there be a macro to avoid
> cut/paste. Perhaps even in the SPI core rather than this driver.

I will post the change for moving the code to core later. I will keep 
this for now.

> > +             struct tegra_slink_data *tspi, struct spi_transfer *t)
> Are there no cases where we can simply DMA straight into the client
> buffer? I suppose it would be limited to cache-line-aligned
> cache-line-size-aligned buffers which is probably unlikely in general:-(

I think it is possible by checking some flags, I will explore and will 
post the fix later.

> > +static int tegra_slink_unprepare_transfer(struct spi_master *master)
> > +{
> > +     struct tegra_slink_data *tspi = spi_master_get_devdata(master);
> > +
> > +     pm_runtime_put_sync(tspi->dev);
> > +     return 0;
> > +}
> Does this driver actually implement any runtime PM ops? I certainly
> don't see any in struct dev_pm_ops tegra_slink_dev_pm_ops. Related, why
> do tegra_slink_clk_{en,dis}able exist; shouldn't those functions be the
> implementation of the runtime PM ops? Related, why are
> tegra_slink_clk_{en,dis}able called all over the place, rather than
> relying on runtime PM API calls, or tegra_slink_{,un}prepare_transfer
> having been called?


OK, I move the clock control in runtime callbacks.


> So that all implies that we wait for the very first interrupt from the
> SPI peripheral for a transfer, and then wait for the DMA controller to
> complete all DMA (which would probably entail actually waiting for DMA
> to drain the RX FIFO in the RX case). Does it make sense to simply
> return from the ISR if not all conditions that we will wait on have
> occurred, and so avoid blocking this ISR thread? I suppose since this
> thread gets blocked rather than spinning, it's not a big deal though.

In this case, we can get rid of isr thread and move the wait/status 
check to caller thread.
I need to do some more stress on this and if it is fine then will post 
the patch later for this, not as part of this patch.


> > +     spin_lock_irqsave(&tspi->lock, flags);
> > +     if (err) {
> > +             dev_err(tspi->dev,
> > +                     "DmaXfer: ERROR bit set 0x%x\n", tspi->status_reg);
> > +             dev_err(tspi->dev,
> > +                     "DmaXfer 0x%08x:0x%08x:0x%08x\n", tspi->command_reg,
> > +                             tspi->command2_reg, tspi->dma_control_reg);
> > +             tegra_periph_reset_assert(tspi->clk);
> > +             udelay(2);
> > +             tegra_periph_reset_deassert(tspi->clk);
> Do we /have/ to reset the SPI block; can't we just disable it in the
> control register, clear all status, and re-program it from scratch?
>
> If at all possible, I would like to avoid introducing any new use of
> tegra_periph_reset_{,de}assert, since that API has no standard subsystem
> equivalent (or if it does, isn't hooked into the standard subsystem
> yet), and hence means this driver relies on a header file currently in
> arch/arm/mach-tegra/include/mach, and we need to move or delete all such
> headers in order to support single zImage.

Is there a way to support the reset of controller. We will need this 
functionality.

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