Re: [PATCH v3 2/5] PCI: dwc: Skip finding eDMA channels count if glue drivers have passed them

[Serge Semin <fancer.lancer@gmail.com>]

On Tue, Feb 27, 2024 at 02:12:04PM +0530, Manivannan Sadhasivam wrote:
> On Tue, Feb 27, 2024 at 12:32:44AM +0300, Serge Semin wrote:
> > On Mon, Feb 26, 2024 at 09:00:14PM +0530, Manivannan Sadhasivam wrote:
> > > On Mon, Feb 26, 2024 at 03:53:20PM +0300, Serge Semin wrote:
> > > > On Mon, Feb 26, 2024 at 05:07:27PM +0530, Manivannan Sadhasivam wrote:
> > > > > In the case of Hyper DMA (HDMA) present in DWC controllers, there is no way
> > > > > the drivers can auto detect the number of read/write channels as like its
> > > > > predecessor embedded DMA (eDMA). So the glue drivers making use of HDMA
> > > > > have to pass the channels count during probe.
> > > > > 
> > > > > To accommodate that, let's skip finding the channels if the channels count
> > > > > were already passed by glue drivers. If the channels count passed were
> > > > > wrong in any form, then the existing sanity check will catch it.
> > > > > 
> > > > > Suggested-by: Serge Semin <fancer.lancer@gmail.com>
> > > > > Reviewed-by: Siddharth Vadapalli <s-vadapalli@ti.com>
> > > > > Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
> > > > > ---
> > > > >  drivers/pci/controller/dwc/pcie-designware.c | 16 +++++++++-------
> > > > >  1 file changed, 9 insertions(+), 7 deletions(-)
> > > > > 
> > > > > diff --git a/drivers/pci/controller/dwc/pcie-designware.c b/drivers/pci/controller/dwc/pcie-designware.c
> > > > > index 193fcd86cf93..ce273c3c5421 100644
> > > > > --- a/drivers/pci/controller/dwc/pcie-designware.c
> > > > > +++ b/drivers/pci/controller/dwc/pcie-designware.c
> > > > > @@ -927,13 +927,15 @@ static int dw_pcie_edma_find_channels(struct dw_pcie *pci)
> > > > >  {
> > > > >  	u32 val;
> > > > >  
> > > > > -	if (pci->edma.mf == EDMA_MF_EDMA_LEGACY)
> > > > > -		val = dw_pcie_readl_dbi(pci, PCIE_DMA_VIEWPORT_BASE + PCIE_DMA_CTRL);
> > > > > -	else
> > > > > -		val = dw_pcie_readl_dma(pci, PCIE_DMA_CTRL);
> > > > > -
> > > > > -	pci->edma.ll_wr_cnt = FIELD_GET(PCIE_DMA_NUM_WR_CHAN, val);
> > > > > -	pci->edma.ll_rd_cnt = FIELD_GET(PCIE_DMA_NUM_RD_CHAN, val);
> > > > 
> > > > > +	if (!pci->edma.ll_wr_cnt || !pci->edma.ll_rd_cnt) {
> > > > 
> > > > Are you sure that the partly initialized case should be considered as
> > > > a request for the auto-detection? IMO &&-ing here and letting the
> > > > sanity check to fail further would be more correct since thus the
> > > > developer would know about improper initialized data.
> > > > 
> > > 
> > > We already have the check below. So the partly initialized case will fail
> > > anyway.
> > 
> > Not really. If the partly initialized case activates the
> > auto-detection procedure it will override both non-initialized and
> > _initialized_ number of channels with the values retrieved from the
> > hardware, which the glue driver has been willing not to use. This
> > prone to undefined behavior depending on the reasons of skipping the
> > auto-detection procedure. For instance, assume the DMA_CTRL register
> > reports an invalid number of read channels. A glue driver by mistake
> > or willingly overwrites the pci->edma.ll_rd_cnt field only. This won't
> > solve the problem since the auto-detection will be proceeded due to
> > the pci->edma.ll_wr_cnt field being left uninitialized.
> > 
> > So to speak it would be better to implement a strictly determined case
> > for activating the auto-detection procedure: both number of channels
> > aren't initialized; if only one field is initialized then report an
> > error.
> > 
> > Alternatively we can have the auto-detection executed on the
> > per-channel basis:
> > 
> > +	if (pci->edma.mf != EDMA_MF_HDMA_NATIVE) {
> > +		val = dw_pcie_readl_dma(pci, PCIE_DMA_CTRL);
> > +
> > +		if (!pci->edma.ll_wr_cnt)
> > +			pci->edma.ll_wr_cnt = FIELD_GET(PCIE_DMA_NUM_WR_CHAN, val);
> > +
> > +		if (!pci->edma.ll_rd_cnt)
> > +			pci->edma.ll_rd_cnt = FIELD_GET(PCIE_DMA_NUM_RD_CHAN, val);
> > +	}
> > 
> 
> Hmm, in this case there is no need to check for uninitialized channels count:
> 
> 	/*
> 	 * Autodetect the read/write channels count only for non-HDMA platforms.
> 	 * HDMA platforms doesn't support autodetect, so the glue drivers should've
> 	 * passed the valid count already. If not, the below sanity check will
> 	 * catch it.
> 	 */
> 	if (pci->edma.mf != EDMA_MF_HDMA_NATIVE) {
> 		val = dw_pcie_readl_dma(pci, PCIE_DMA_CTRL);
> 
> 		pci->edma.ll_wr_cnt = FIELD_GET(PCIE_DMA_NUM_WR_CHAN, val);
> 		pci->edma.ll_rd_cnt = FIELD_GET(PCIE_DMA_NUM_RD_CHAN, val);
> 	}
> 
> 	/* Sanity check */

That is another possible implementation. Let's sum all of them up:

1. Channel fields-base conditional statement:
+	if (!pci->edma.ll_wr_cnt && !pci->edma.ll_rd_cnt) {
+		val = dw_pcie_readl_dma(pci, PCIE_DMA_CTRL);
+
+		pci->edma.ll_wr_cnt = FIELD_GET(PCIE_DMA_NUM_WR_CHAN, val);
+		pci->edma.ll_rd_cnt = FIELD_GET(PCIE_DMA_NUM_RD_CHAN, val);
+	}
pros: NoF channels override support for all IP-cores; simple.
cons: incompatible with HDMA, but can be taken by mistake/bug; no
partial NoF channels pre-initialization.

2. Channel fields-base conditional statement with logical OR operator #1:
+	if (!pci->edma.ll_wr_cnt || !pci->edma.ll_rd_cnt) {
+		val = dw_pcie_readl_dma(pci, PCIE_DMA_CTRL);
+
+		pci->edma.ll_wr_cnt = FIELD_GET(PCIE_DMA_NUM_WR_CHAN, val);
+		pci->edma.ll_rd_cnt = FIELD_GET(PCIE_DMA_NUM_RD_CHAN, val);
+	}
pros: NoF channels override support for all IP-cores; simple.
cons: incompatible with HDMA, but can be taken by mistake/bug; no
partial NoF channels pre-initialization; silently overrides the
partial NoF channels case.

3. Channel fields-base conditional statement with logical OR operator #2:
+	if (!pci->edma.ll_wr_cnt || !pci->edma.ll_rd_cnt) {
+		val = dw_pcie_readl_dma(pci, PCIE_DMA_CTRL);
+
+		if (!pci->edma.ll_wr_cnt)
+			pci->edma.ll_wr_cnt = FIELD_GET(PCIE_DMA_NUM_WR_CHAN, val);
+
+		if (!pci->edma.ll_rd_cnt)
+			pci->edma.ll_rd_cnt = FIELD_GET(PCIE_DMA_NUM_RD_CHAN, val);
+	}
pros: NoF channels override support for all IP-cores; partial NoF
channels pre-initialization support.
cons: incompatible with HDMA, but can be taken by mistake/bug; more
complex (and actually looking a bit clumsy due to two conditional
statements over the same fields).

4. Unconditional auto-detection:
+	val = dw_pcie_readl_dma(pci, PCIE_DMA_CTRL);
+
+	if (!pci->edma.ll_wr_cnt)
+		pci->edma.ll_wr_cnt = FIELD_GET(PCIE_DMA_NUM_WR_CHAN, val);
+
+	if (!pci->edma.ll_rd_cnt)
+		pci->edma.ll_rd_cnt = FIELD_GET(PCIE_DMA_NUM_RD_CHAN, val);
pros: NoF channels override support for all IP-cores; partial NoF
channels pre-initialization support; simple.
cons: incompatible with HDMA, but will be executed for it anyway so
the NoF channels fields will be overridden with the Channel#0.prefetch
CSR data if haven't been pre-initialized;

5. Mapping format-based conditional statement:
+	if (pci->edma.mf != EDMA_MF_HDMA_NATIVE) {
+		val = dw_pcie_readl_dma(pci, PCIE_DMA_CTRL);
+
+		pci->edma.ll_wr_cnt = FIELD_GET(PCIE_DMA_NUM_WR_CHAN, val);
+		pci->edma.ll_rd_cnt = FIELD_GET(PCIE_DMA_NUM_RD_CHAN, val);
+	}
pros: free of being executed for HDMA IP-core, simple
cons: no NoF channels override support for non-HDMA IP-cores.

6. Mapping format-based conditional statement with partial NoF channels override:
+	if (pci->edma.mf != EDMA_MF_HDMA_NATIVE) {
+		val = dw_pcie_readl_dma(pci, PCIE_DMA_CTRL);
+
+		if (!pci->edma.ll_wr_cnt)
+			pci->edma.ll_wr_cnt = FIELD_GET(PCIE_DMA_NUM_WR_CHAN, val);
+
+		if (!pci->edma.ll_rd_cnt)
+			pci->edma.ll_rd_cnt = FIELD_GET(PCIE_DMA_NUM_RD_CHAN, val);
+	}
pros: free of being executed for HDMA IP-core; NoF channels override
support for all IP-cores.
cons: more complex.


Looking at all of that I'd say that options 5 and 6 seems better to me
now since they prohibit the auto-detection for HDMA IP-cores which
have the Channel#0.prefetch CSR at the 0x8 offset. I don't have strong
opinion which of those two to choose. If you think simplicity is
preferable, then option 2 will be enough. If you wish to have the NoF
channels override supported for all IP-cores, then option 3 will work
for it.

-Serge(y)

> 
> - Mani
> 
> -- 
> மணிவண்ணன் சதாசிவம்