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

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Message-ID: <20240227084204.GI2587@thinkpad>
Date: Tue, 27 Feb 2024 14:12:04 +0530
From: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
To: Serge Semin <fancer.lancer@...il.com>
Cc: Jingoo Han <jingoohan1@...il.com>,
	Gustavo Pimentel <gustavo.pimentel@...opsys.com>,
	Lorenzo Pieralisi <lpieralisi@...nel.org>,
	Krzysztof Wilczyński <kw@...ux.com>,
	Rob Herring <robh@...nel.org>, Bjorn Helgaas <bhelgaas@...gle.com>,
	Marek Vasut <marek.vasut+renesas@...il.com>,
	Yoshihiro Shimoda <yoshihiro.shimoda.uh@...esas.com>,
	Kishon Vijay Abraham I <kishon@...nel.org>,
	linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-renesas-soc@...r.kernel.org, linux-arm-msm@...r.kernel.org,
	mhi@...ts.linux.dev, Siddharth Vadapalli <s-vadapalli@...com>
Subject: Re: [PATCH v3 2/5] PCI: dwc: Skip finding eDMA channels count if
 glue drivers have passed them

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@...il.com>
> > > > Reviewed-by: Siddharth Vadapalli <s-vadapalli@...com>
> > > > Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.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 */

- Mani

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