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Date: Fri, 5 Jun 2020 12:34:36 +0100
From: Robin Murphy <robin.murphy@....com>
To: Florian Fainelli <f.fainelli@...il.com>,
linux-kernel@...r.kernel.org
Cc: "moderated list:BROADCOM BCM2711/BCM2835 ARM ARCHITECTURE"
<linux-arm-kernel@...ts.infradead.org>,
"open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS"
<devicetree@...r.kernel.org>,
Scott Branden <sbranden@...adcom.com>, lukas@...ner.de,
Ray Jui <rjui@...adcom.com>, Rob Herring <robh+dt@...nel.org>,
"open list:SPI SUBSYSTEM" <linux-spi@...r.kernel.org>,
Mark Brown <broonie@...nel.org>,
"maintainer:BROADCOM BCM281XX/BCM11XXX/BCM216XX ARM ARCHITE..."
<bcm-kernel-feedback-list@...adcom.com>,
"moderated list:BROADCOM BCM2711/BCM2835 ARM ARCHITECTURE"
<linux-rpi-kernel@...ts.infradead.org>,
Martin Sperl <kernel@...tin.sperl.org>,
Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
Subject: Re: [PATCH v2] spi: bcm2835: Enable shared interrupt support
On 2020-06-04 22:28, Florian Fainelli wrote:
> The 4 SPI controller instances added in BCM2711 and BCM7211 SoCs (SPI3,
> SPI4, SPI5 and SPI6) share the same interrupt line with SPI0.
>
> For the BCM2835 case which is deemed performance critical, we would like
> to continue using an interrupt handler which does not have the extra
> comparison on BCM2835_SPI_CS_INTR.
FWIW, if I'm reading the patch correctly, then with sensible codegen
that "overhead" should amount to a bit test on a live register plus a
not-taken conditional branch - according to the 1176 TRM that should add
up to a whopping 2 cycles. If that's really significant then I'd have to
wonder whether you want to be at the mercy of the whole generic IRQ
stack at all, and should perhaps consider using FIQ instead.
> To support that requirement the common interrupt handling code between
> the shared and non-shared interrupt paths is split into a
> bcm2835_spi_interrupt_common() and both bcm2835_spi_interrupt() as well
> as bcm2835_spi_shared_interrupt() make use of it.
>
> During probe, we determine if there is at least another instance of this
> SPI controller, and if there is, then we install a shared interrupt
> handler.
>
> Signed-off-by: Florian Fainelli <f.fainelli@...il.com>
> ---
> Changes in v2:
>
> - identify other available SPI nodes to determine if we need to set-up
> interrupt sharing. This needs to happen for the very first instance
> since we cannot know for the first instance whether interrupt sharing
> is needed or not.
>
> drivers/spi/spi-bcm2835.c | 61 ++++++++++++++++++++++++++++++++-------
> 1 file changed, 50 insertions(+), 11 deletions(-)
>
> diff --git a/drivers/spi/spi-bcm2835.c b/drivers/spi/spi-bcm2835.c
> index 237bd306c268..0288b5b3de1e 100644
> --- a/drivers/spi/spi-bcm2835.c
> +++ b/drivers/spi/spi-bcm2835.c
> @@ -361,11 +361,10 @@ static void bcm2835_spi_reset_hw(struct spi_controller *ctlr)
> bcm2835_wr(bs, BCM2835_SPI_DLEN, 0);
> }
>
> -static irqreturn_t bcm2835_spi_interrupt(int irq, void *dev_id)
> +static inline irqreturn_t bcm2835_spi_interrupt_common(struct spi_controller *ctlr,
> + u32 cs)
> {
> - struct spi_controller *ctlr = dev_id;
> struct bcm2835_spi *bs = spi_controller_get_devdata(ctlr);
> - u32 cs = bcm2835_rd(bs, BCM2835_SPI_CS);
>
> /*
> * An interrupt is signaled either if DONE is set (TX FIFO empty)
> @@ -394,6 +393,27 @@ static irqreturn_t bcm2835_spi_interrupt(int irq, void *dev_id)
> return IRQ_HANDLED;
> }
>
> +static irqreturn_t bcm2835_spi_interrupt(int irq, void *dev_id)
> +{
> + struct spi_controller *ctlr = dev_id;
> + struct bcm2835_spi *bs = spi_controller_get_devdata(ctlr);
> + u32 cs = bcm2835_rd(bs, BCM2835_SPI_CS);
> +
> + return bcm2835_spi_interrupt_common(ctlr, cs);
> +}
> +
> +static irqreturn_t bcm2835_spi_shared_interrupt(int irq, void *dev_id)
> +{
> + struct spi_controller *ctlr = dev_id;
> + struct bcm2835_spi *bs = spi_controller_get_devdata(ctlr);
> + u32 cs = bcm2835_rd(bs, BCM2835_SPI_CS);
> +
> + if (!(cs & BCM2835_SPI_CS_INTR))
> + return IRQ_NONE;
> +
> + return bcm2835_spi_interrupt_common(ctlr, cs);
> +}
> +
> static int bcm2835_spi_transfer_one_irq(struct spi_controller *ctlr,
> struct spi_device *spi,
> struct spi_transfer *tfr,
> @@ -1287,12 +1307,37 @@ static int bcm2835_spi_setup(struct spi_device *spi)
> return 0;
> }
>
> +static const struct of_device_id bcm2835_spi_match[] = {
> + { .compatible = "brcm,bcm2835-spi", },
> + {}
> +};
> +MODULE_DEVICE_TABLE(of, bcm2835_spi_match);
> +
> static int bcm2835_spi_probe(struct platform_device *pdev)
> {
> + irq_handler_t bcm2835_spi_isr_func = bcm2835_spi_interrupt;
> struct spi_controller *ctlr;
> + unsigned long flags = 0;
> + struct device_node *dn;
> struct bcm2835_spi *bs;
> int err;
>
> + /* On BCM2711 there can be multiple SPI controllers enabled sharing the
> + * same interrupt line, but we also want to minimize the overhead if
> + * there is no need to support interrupt sharing. If we find at least
> + * another available instane (not counting the one we are probed from),
> + * then we assume that interrupt sharing is necessary.
> + */
> + for_each_compatible_node(dn, NULL, bcm2835_spi_match[0].compatible) {
> + err = of_device_is_available(dn) && dn != pdev->dev.of_node;
> + of_node_put(dn);
This is in the wrong place - it should only be where you terminate the
loop early and thus bypass the "of_node_put(from)" call in the iterator
itself.
> + if (err) {
> + flags = IRQF_SHARED;
Is there really any harm to setting IRQF_SHARED even when the interrupt
isn't shared in hardware? Sure, it means you lose a degree of API-level
validation and some other driver might also be able to simultaneously
claim it on bcm283x, but in that case the DT would be wrong and
*something* isn't going to work correctly anyway, so does this one
driver really need to care about trying to be the DT police?
Robin.
> + bcm2835_spi_isr_func = bcm2835_spi_shared_interrupt;
> + break;
> + }
> + }
> +
> ctlr = spi_alloc_master(&pdev->dev, ALIGN(sizeof(*bs),
> dma_get_cache_alignment()));
> if (!ctlr)
> @@ -1344,8 +1389,8 @@ static int bcm2835_spi_probe(struct platform_device *pdev)
> bcm2835_wr(bs, BCM2835_SPI_CS,
> BCM2835_SPI_CS_CLEAR_RX | BCM2835_SPI_CS_CLEAR_TX);
>
> - err = devm_request_irq(&pdev->dev, bs->irq, bcm2835_spi_interrupt, 0,
> - dev_name(&pdev->dev), ctlr);
> + err = devm_request_irq(&pdev->dev, bs->irq, bcm2835_spi_isr_func,
> + flags, dev_name(&pdev->dev), ctlr);
> if (err) {
> dev_err(&pdev->dev, "could not request IRQ: %d\n", err);
> goto out_dma_release;
> @@ -1400,12 +1445,6 @@ static void bcm2835_spi_shutdown(struct platform_device *pdev)
> dev_err(&pdev->dev, "failed to shutdown\n");
> }
>
> -static const struct of_device_id bcm2835_spi_match[] = {
> - { .compatible = "brcm,bcm2835-spi", },
> - {}
> -};
> -MODULE_DEVICE_TABLE(of, bcm2835_spi_match);
> -
> static struct platform_driver bcm2835_spi_driver = {
> .driver = {
> .name = DRV_NAME,
>
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