[PATCH 2/2] v1 ARM: sun4i: spi: Allow Tx transfers larger than FIFO size

[PATCH 2/2] v1 ARM: sun4i: spi: Allow Tx transfers larger than FIFO size

[Alexandru Gagniuc]

Enable and use the Tx FIFO 3/4 interrupt to replenish the FIFO on
large SPI bursts. This requires more care in when the interrupt is
left enabled, as this interrupt will continually trigger when the FIFO
is less than 1/4 full, even though we acknowledge it.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
---
 drivers/spi/spi-sun4i.c | 44 ++++++++++++++++++++++++++++++++++++++------
 1 file changed, 38 insertions(+), 6 deletions(-)

diff --git a/drivers/spi/spi-sun4i.c b/drivers/spi/spi-sun4i.c
index 09d4b54..174736c 100644
--- a/drivers/spi/spi-sun4i.c
+++ b/drivers/spi/spi-sun4i.c
@@ -48,6 +48,7 @@
 
 #define SUN4I_INT_CTL_REG		0x0c
 #define SUN4I_INT_CTL_RF_F34			BIT(4)
+#define SUN4I_INT_CTL_TF_E34			BIT(12)
 #define SUN4I_INT_CTL_TC			BIT(16)
 
 #define SUN4I_INT_STA_REG		0x10
@@ -100,6 +101,21 @@ static inline void sun4i_spi_write(struct sun4i_spi 
*sspi, u32 reg, u32 value)
 	writel(value, sspi->base_addr + reg);
 }
 
+static inline int sun4i_spi_get_tx_fifo_count(struct sun4i_spi *sspi)
+{
+	u32 reg;
+	reg = sun4i_spi_read(sspi, SUN4I_FIFO_STA_REG);
+	reg >>= SUN4I_FIFO_STA_TF_CNT_BITS;
+	return reg & SUN4I_FIFO_STA_TF_CNT_MASK;
+}
+
+static inline void sun4i_spi_disable_interrupt(struct sun4i_spi *sspi, u32 
intm)
+{
+	u32 reg = sun4i_spi_read(sspi, SUN4I_INT_CTL_REG);
+	reg &= ~intm;
+	sun4i_spi_write(sspi, SUN4I_INT_CTL_REG, reg);
+}
+
 static inline void sun4i_spi_drain_fifo(struct sun4i_spi *sspi, int len)
 {
 	u32 reg, cnt;
@@ -181,10 +197,6 @@ static int sun4i_spi_transfer_one(struct spi_master 
*master,
 	if (tfr->len > SUN4I_MAX_XFER_SIZE)
 		return -EINVAL;
 
-	/* We only support read transfers larger than the FIFO */
-	if ((tfr->len > SUN4I_FIFO_DEPTH) && tfr->tx_buf)
-		return -EINVAL;
-
 	reinit_completion(&sspi->done);
 	sspi->tx_buf = tfr->tx_buf;
 	sspi->rx_buf = tfr->rx_buf;
@@ -280,8 +292,11 @@ static int sun4i_spi_transfer_one(struct spi_master 
*master,
 	sun4i_spi_fill_fifo(sspi, SUN4I_FIFO_DEPTH);
 
 	/* Enable the interrupts */
-	sun4i_spi_write(sspi, SUN4I_INT_CTL_REG, SUN4I_INT_CTL_TC |
-						 SUN4I_INT_CTL_RF_F34);
+	reg = SUN4I_INT_CTL_TC | SUN4I_INT_CTL_RF_F34;
+	/* Only enable Tx FIFO interrupt if we really need it */
+	if (tx_len > SUN4I_FIFO_DEPTH)
+		reg |= SUN4I_INT_CTL_TF_E34;
+	sun4i_spi_write(sspi, SUN4I_INT_CTL_REG, reg);
 
 	/* Start the transfer */
 	reg = sun4i_spi_read(sspi, SUN4I_CTL_REG);
@@ -306,6 +321,7 @@ static irqreturn_t sun4i_spi_handler(int irq, void 
*dev_id)
 {
 	struct sun4i_spi *sspi = dev_id;
 	u32 status = sun4i_spi_read(sspi, SUN4I_INT_STA_REG);
+	u32 cnt;
 
 	/* Transfer complete */
 	if (status & SUN4I_INT_CTL_TC) {
@@ -323,6 +339,22 @@ static irqreturn_t sun4i_spi_handler(int irq, void 
*dev_id)
 		return IRQ_HANDLED;
 	}
 
+	/* Transmit FIFO 3/4 empty */
+	if (status & SUN4I_INT_CTL_TF_E34) {
+		/* See how much data can fit */
+		cnt = SUN4I_FIFO_DEPTH - sun4i_spi_get_tx_fifo_count(sspi);
+		sun4i_spi_fill_fifo(sspi, cnt);
+
+		if(!sspi->len)
+			/* nothing left to transmit */
+			sun4i_spi_disable_interrupt(sspi, 
SUN4I_INT_CTL_TF_E34);
+
+		/* Only clear the interrupt _after_ re-seeding the FIFO */
+		sun4i_spi_write(sspi, SUN4I_INT_STA_REG, 
SUN4I_INT_CTL_TF_E34);
+
+		return IRQ_HANDLED;
+	}
+
 	return IRQ_NONE;
 }
 
-- 
1.8.5.3

Re: [PATCH 2/2] v1 ARM: sun4i: spi: Allow Tx transfers larger than FIFO size

[Maxime Ripard]

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On Tue, Mar 18, 2014 at 05:04:36PM -0500, Alexandru Gagniuc wrote:
> Enable and use the Tx FIFO 3/4 interrupt to replenish the FIFO on
> large SPI bursts. This requires more care in when the interrupt is
> left enabled, as this interrupt will continually trigger when the FIFO
> is less than 1/4 full, even though we acknowledge it.
> 
> Signed-off-by: Alexandru Gagniuc <mr.nuke.me-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
> ---
>  drivers/spi/spi-sun4i.c | 44 ++++++++++++++++++++++++++++++++++++++------
>  1 file changed, 38 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/spi/spi-sun4i.c b/drivers/spi/spi-sun4i.c
> index 09d4b54..174736c 100644
> --- a/drivers/spi/spi-sun4i.c
> +++ b/drivers/spi/spi-sun4i.c
> @@ -48,6 +48,7 @@
>  
>  #define SUN4I_INT_CTL_REG		0x0c
>  #define SUN4I_INT_CTL_RF_F34			BIT(4)
> +#define SUN4I_INT_CTL_TF_E34			BIT(12)
>  #define SUN4I_INT_CTL_TC			BIT(16)
>  
>  #define SUN4I_INT_STA_REG		0x10
> @@ -100,6 +101,21 @@ static inline void sun4i_spi_write(struct sun4i_spi 
> *sspi, u32 reg, u32 value)
>  	writel(value, sspi->base_addr + reg);
>  }
>  
> +static inline int sun4i_spi_get_tx_fifo_count(struct sun4i_spi *sspi)

return an u32 here

> +{
> +	u32 reg;
> +	reg = sun4i_spi_read(sspi, SUN4I_FIFO_STA_REG);
> +	reg >>= SUN4I_FIFO_STA_TF_CNT_BITS;
> +	return reg & SUN4I_FIFO_STA_TF_CNT_MASK;
> +}
> +
> +static inline void sun4i_spi_disable_interrupt(struct sun4i_spi *sspi, u32 
> intm)

Please rename the variable "mask" here, and add a matching
enable_interrupt function for consistency.

> +{
> +	u32 reg = sun4i_spi_read(sspi, SUN4I_INT_CTL_REG);
> +	reg &= ~intm;
> +	sun4i_spi_write(sspi, SUN4I_INT_CTL_REG, reg);
> +}
> +
>  static inline void sun4i_spi_drain_fifo(struct sun4i_spi *sspi, int len)
>  {
>  	u32 reg, cnt;
> @@ -181,10 +197,6 @@ static int sun4i_spi_transfer_one(struct spi_master 
> *master,
>  	if (tfr->len > SUN4I_MAX_XFER_SIZE)
>  		return -EINVAL;
>  
> -	/* We only support read transfers larger than the FIFO */
> -	if ((tfr->len > SUN4I_FIFO_DEPTH) && tfr->tx_buf)
> -		return -EINVAL;
> -
>  	reinit_completion(&sspi->done);
>  	sspi->tx_buf = tfr->tx_buf;
>  	sspi->rx_buf = tfr->rx_buf;
> @@ -280,8 +292,11 @@ static int sun4i_spi_transfer_one(struct spi_master 
> *master,
>  	sun4i_spi_fill_fifo(sspi, SUN4I_FIFO_DEPTH);
>  
>  	/* Enable the interrupts */
> -	sun4i_spi_write(sspi, SUN4I_INT_CTL_REG, SUN4I_INT_CTL_TC |
> -						 SUN4I_INT_CTL_RF_F34);
> +	reg = SUN4I_INT_CTL_TC | SUN4I_INT_CTL_RF_F34;
> +	/* Only enable Tx FIFO interrupt if we really need it */
> +	if (tx_len > SUN4I_FIFO_DEPTH)
> +		reg |= SUN4I_INT_CTL_TF_E34;
> +	sun4i_spi_write(sspi, SUN4I_INT_CTL_REG, reg);

I'd be much dumber than that. Why don't you just enable both
interrupts all the time if we need larger transfers ?


>  	/* Start the transfer */
>  	reg = sun4i_spi_read(sspi, SUN4I_CTL_REG);
> @@ -306,6 +321,7 @@ static irqreturn_t sun4i_spi_handler(int irq, void 
> *dev_id)
>  {
>  	struct sun4i_spi *sspi = dev_id;
>  	u32 status = sun4i_spi_read(sspi, SUN4I_INT_STA_REG);
> +	u32 cnt;
>  
>  	/* Transfer complete */
>  	if (status & SUN4I_INT_CTL_TC) {
> @@ -323,6 +339,22 @@ static irqreturn_t sun4i_spi_handler(int irq, void 
> *dev_id)
>  		return IRQ_HANDLED;
>  	}
>  
> +	/* Transmit FIFO 3/4 empty */
> +	if (status & SUN4I_INT_CTL_TF_E34) {
> +		/* See how much data can fit */
> +		cnt = SUN4I_FIFO_DEPTH - sun4i_spi_get_tx_fifo_count(sspi);
> +		sun4i_spi_fill_fifo(sspi, cnt);

Making sure that you don't write to much data should be part of
_fill_fifo

Maxime

-- 
Maxime Ripard, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com

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Re: [PATCH 2/2] v1 ARM: sun4i: spi: Allow Tx transfers larger than FIFO size

[mrnuke]

On Wednesday, March 19, 2014 06:02:51 PM Maxime Ripard wrote:

> > +static inline int sun4i_spi_get_tx_fifo_count(struct sun4i_spi *sspi)
> 
> return an u32 here
> 
> > +static inline void sun4i_spi_disable_interrupt(struct sun4i_spi *sspi,
> > u32
> > intm)
> 
> Please rename the variable "mask" here, and add a matching
> enable_interrupt function for consistency.
>
OK, and OK.

> >  	/* Enable the interrupts */
> > 
> > -	sun4i_spi_write(sspi, SUN4I_INT_CTL_REG, SUN4I_INT_CTL_TC |
> > -						 SUN4I_INT_CTL_RF_F34);
> > +	reg = SUN4I_INT_CTL_TC | SUN4I_INT_CTL_RF_F34;
> > +	/* Only enable Tx FIFO interrupt if we really need it */
> > +	if (tx_len > SUN4I_FIFO_DEPTH)
> > +		reg |= SUN4I_INT_CTL_TF_E34;
> > +	sun4i_spi_write(sspi, SUN4I_INT_CTL_REG, reg);
> 
> I'd be much dumber than that. Why don't you just enable both
> interrupts all the time if we need larger transfers ?
> 
SUN4I_INT_CTL_TF_E34 will trigger whenever the Tx FIFO has 16 bytes or less. 
There are 2 cases where this is relevant:

 (a) We have a Tx transaction with a length less than the FIFO size. We start 
the transaction, and SUN4I_INT_CTL_TF_E34 gets triggered somewhere along the 
way. We enter the interrupt handler, see that sspi->len is 0, and disable this 
interrupt.

 (b) We have a long Rx-only transaction. We start the transaction, and 
SUN4I_INT_CTL_TF_E34 gets triggered right away, as our Tx buffer is empty. We 
enter the handler, see that sspi->len is not zero, so we leave the interrupt 
enabled. Exit the IRQ handler, and we're right back servicing the same 
interrupt.

Case (a) only costs us an extra interrupt, whereas case (b) will cause an IRQ 
storm, and essentially loop-brick the kernel. I think the current check is the 
simplest of the alternatives. It's also why I wanted to separate the Tx part 
in a separate patch. This interrupt is tricky.

> > +	/* Transmit FIFO 3/4 empty */
> > +	if (status & SUN4I_INT_CTL_TF_E34) {
> > +		/* See how much data can fit */
> > +		cnt = SUN4I_FIFO_DEPTH - sun4i_spi_get_tx_fifo_count(sspi);
> > +		sun4i_spi_fill_fifo(sspi, cnt);
> 
> Making sure that you don't write to much data should be part of
> _fill_fifo
> 
I thought about it and the symmetry with _drain_fifo. I think this makes it a 
little clearer to see what's going on, but I'm perfectly fine with the 
symmetric way.

Alex
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Re: [PATCH 2/2] v1 ARM: sun4i: spi: Allow Tx transfers larger than FIFO size

[Maxime Ripard]

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On Wed, Mar 19, 2014 at 01:23:34PM -0500, mrnuke wrote:
> > >  	/* Enable the interrupts */
> > > 
> > > -	sun4i_spi_write(sspi, SUN4I_INT_CTL_REG, SUN4I_INT_CTL_TC |
> > > -						 SUN4I_INT_CTL_RF_F34);
> > > +	reg = SUN4I_INT_CTL_TC | SUN4I_INT_CTL_RF_F34;
> > > +	/* Only enable Tx FIFO interrupt if we really need it */
> > > +	if (tx_len > SUN4I_FIFO_DEPTH)
> > > +		reg |= SUN4I_INT_CTL_TF_E34;
> > > +	sun4i_spi_write(sspi, SUN4I_INT_CTL_REG, reg);
> > 
> > I'd be much dumber than that. Why don't you just enable both
> > interrupts all the time if we need larger transfers ?
> > 
> SUN4I_INT_CTL_TF_E34 will trigger whenever the Tx FIFO has 16 bytes or less. 
> There are 2 cases where this is relevant:
> 
>  (a) We have a Tx transaction with a length less than the FIFO size. We start 
> the transaction, and SUN4I_INT_CTL_TF_E34 gets triggered somewhere along the 
> way. We enter the interrupt handler, see that sspi->len is 0, and disable this 
> interrupt.

Well, it wouldn't be triggered, since you wouldn't have enabled
it. But it would be in the status register.

>  (b) We have a long Rx-only transaction. We start the transaction, and 
> SUN4I_INT_CTL_TF_E34 gets triggered right away, as our Tx buffer is empty. We 
> enter the handler, see that sspi->len is not zero, so we leave the interrupt 
> enabled. Exit the IRQ handler, and we're right back servicing the same 
> interrupt.

Ah, yes. That one is nasty.

> Case (a) only costs us an extra interrupt, whereas case (b) will cause an IRQ 
> storm, and essentially loop-brick the kernel. I think the current check is the 
> simplest of the alternatives. It's also why I wanted to separate the Tx part 
> in a separate patch. This interrupt is tricky.

I guess your suggestion of adding an enabled interrupt mask makes
sense then, just to avoid handling status that we don't care for in
the case of a spurious interrupt.

-- 
Maxime Ripard, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com

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