linux-kernel - Re: [PATCH 1/4] spi: lantiq: Synchronize interrupt handlers and transfers

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Message-ID: <20200427134555.GC4383@sirena.org.uk>
Date:   Mon, 27 Apr 2020 14:45:55 +0100
From:   Mark Brown <broonie@...nel.org>
To:     Dilip Kota <eswara.kota@...ux.intel.com>
Cc:     robh@...nel.org, linux-spi@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        daniel.schwierzeck@...il.com, hauke@...ke-m.de,
        andriy.shevchenko@...el.com, cheol.yong.kim@...el.com,
        chuanhua.lei@...ux.intel.com, qi-ming.wu@...el.com
Subject: Re: [PATCH 1/4] spi: lantiq: Synchronize interrupt handlers and
 transfers

On Mon, Apr 27, 2020 at 02:01:29PM +0800, Dilip Kota wrote:
> On 4/24/2020 7:25 PM, Mark Brown wrote:
> > On Fri, Apr 24, 2020 at 06:42:30PM +0800, Dilip Kota wrote:

> > > Synchronize tx, rx and error interrupts by registering to the
> > > same interrupt handler. Interrupt handler will recognize and process
> > > the appropriate interrupt on the basis of interrupt status register.
> > > Also, establish synchronization between the interrupt handler and
> > > transfer operation by taking the locks and registering the interrupt
> > > handler as thread IRQ which avoids the bottom half.
> > > Fixes the wrongly populated interrupt register offsets too.

> > This sounds like at least three different changes mixed together in one
> > commit, it makes it quite hard to tell what's going on.  If nothing else
> > the conversion from a workqueue to threaded interrupts should probably
> > be split out from merging the interrupts.

> While preparing the patches, i got puzzled to go with separate patches (for
> threaded interrupts, unified interrupt handler and fixing the register
> offset) or as a single patch!!.

> Finally i choose to go with single patch, because establishing
> synchronization is the major reason for this change, for that reason
> threaded interrupts and unified interrupts changes are done. And the fixing
> offset is a single line change, so included in this patch itself. And, on a
> lighter note, the whole patch is coming under 45 lines of code changes.
> Please let me know your view.

The single line change to fix the offset sounds like an especially good
candidate for splitting out as a separate patch.  It's not really about
the number of lines but rather complexity.

> > > -static irqreturn_t lantiq_ssc_err_interrupt(int irq, void *data)
> > > +static irqreturn_t lantiq_ssc_err_interrupt(struct lantiq_ssc_spi *spi)
> > >   {
> > > -	struct lantiq_ssc_spi *spi = data;
> > >   	u32 stat = lantiq_ssc_readl(spi, LTQ_SPI_STAT);
> > > -	if (!(stat & LTQ_SPI_STAT_ERRORS))
> > > -		return IRQ_NONE;
> > > -

> > Why drop this?

> lantiq_ssc_err_interrupt() getting called, only if LTQ_SPI_IRNEN_E is set in
> the interrupt status register.
> Once the 'LTQ_SPI_IRNEN_E' bit is set, there is no chance of all error bits
> being unset in the SPI_STAT register, so the 'if condition' will never be
> successful. Hence dropped it.

So this is another separate change and TBH it doesn't seem like a huge
win in that it's still potentially adding a bit of robustness.

> > It's not clear to me that it's a benefit to combine all the interrupts
> > unconditionally - obviously where they're shared we need to but could
> > that be accomplished with IRQF_SHARED and even if it can't it seems like
> > something conditional would be better.

> Lets take a case where Tx/Rx transfer interrupt got triggered and followed
> by error interrupt(before finishing the tx/rx interrupt execution) which is
> very less likely to occur, unified interrupt handler establishes
> synchronization.
> Comparatively, unified interrupt handler is better for adding support to the
> latest SoCs on which SPI have single interrupt line for tx,rx and errors.
> On basis of these two points i felt to go with unified interrupt handler.

Does the mutex not do this regardless of how the interrupt handlers are
wired up?

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