Subject: Re: [PATCH 1/2] i2c: qcom-geni: Provide an option to select FIFO processing

From: Wolfram Sang <[email-masked]>
Date: Wed, 4 Sep 2019 23:23:37 +0200

On Wed, Sep 04, 2019 at 01:35:48PM -0700, Bjorn Andersson wrote:
> On Wed 04 Sep 04:36 PDT 2019, Lee Jones wrote:
> 
> The subject implies that we select FIFO mode instead of DMA, but that's
> not really true, because with DMA enabled we still fall back to FIFO for
> messages below 32 bytes. 
> 
> So what this does it to disable DMA, which neither the subject or the DT
> property describes.
> 
> Also missing is a description of why this is needed.

Yes.

I am willing to help to get this resolved soonish. However, I have
issues with the approach.

It looks like a workaround to me. It would be interesting to hear which
I2C client breaks with DMA and if it's driver can't be fixed somehow
instead. But even if we agree on a workaround short term, adding a
binding for this workaround seems like a no-go to me. We have to live
with this binding forever. Sidenote: I could think of a generic
'disable-dma' which could be reused everywhere but we probably won't get
that upstream that late in the cycle.

Is there no other way to disable DMA which is local to this driver so we
can easily revert the workaround later?

> 
> Regards,
> Bjorn
> 
> > Signed-off-by: Lee Jones <[email-masked]>
> > ---
> >  drivers/i2c/busses/i2c-qcom-geni.c | 14 ++++++++++----
> >  1 file changed, 10 insertions(+), 4 deletions(-)
> > 
> > diff --git a/drivers/i2c/busses/i2c-qcom-geni.c b/drivers/i2c/busses/i2c-qcom-geni.c
> > index a89bfce5388e..dfdbce067827 100644
> > --- a/drivers/i2c/busses/i2c-qcom-geni.c
> > +++ b/drivers/i2c/busses/i2c-qcom-geni.c
> > @@ -353,13 +353,16 @@ static void geni_i2c_tx_fsm_rst(struct geni_i2c_dev *gi2c)
> >  static int geni_i2c_rx_one_msg(struct geni_i2c_dev *gi2c, struct i2c_msg *msg,
> >  				u32 m_param)
> >  {
> > +	struct device_node *np = gi2c->se.dev->of_node;
> >  	dma_addr_t rx_dma;
> >  	unsigned long time_left;
> > -	void *dma_buf;
> > +	void *dma_buf = NULL;
> >  	struct geni_se *se = &gi2c->se;
> >  	size_t len = msg->len;
> >  
> > -	dma_buf = i2c_get_dma_safe_msg_buf(msg, 32);
> > +	if (!of_property_read_bool(np, "qcom,geni-se-fifo"))
> > +		dma_buf = i2c_get_dma_safe_msg_buf(msg, 32);
> > +
> >  	if (dma_buf)
> >  		geni_se_select_mode(se, GENI_SE_DMA);
> >  	else
> > @@ -392,13 +395,16 @@ static int geni_i2c_rx_one_msg(struct geni_i2c_dev *gi2c, struct i2c_msg *msg,
> >  static int geni_i2c_tx_one_msg(struct geni_i2c_dev *gi2c, struct i2c_msg *msg,
> >  				u32 m_param)
> >  {
> > +	struct device_node *np = gi2c->se.dev->of_node;
> >  	dma_addr_t tx_dma;
> >  	unsigned long time_left;
> > -	void *dma_buf;
> > +	void *dma_buf = NULL;
> >  	struct geni_se *se = &gi2c->se;
> >  	size_t len = msg->len;
> >  
> > -	dma_buf = i2c_get_dma_safe_msg_buf(msg, 32);
> > +	if (!of_property_read_bool(np, "qcom,geni-se-fifo"))
> > +		dma_buf = i2c_get_dma_safe_msg_buf(msg, 32);
> > +
> >  	if (dma_buf)
> >  		geni_se_select_mode(se, GENI_SE_DMA);
> >  	else
> > -- 
> > 2.17.1
> > 
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