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Date: Tue, 27 Feb 2024 00:00:41 +0300
From: Serge Semin <fancer.lancer@gmail.com>
To: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Cc: Siddharth Vadapalli <s-vadapalli@ti.com>, 
	Jingoo Han <jingoohan1@gmail.com>, Gustavo Pimentel <gustavo.pimentel@synopsys.com>, 
	Lorenzo Pieralisi <lpieralisi@kernel.org>, Krzysztof =?utf-8?Q?Wilczy=C5=84ski?= <kw@linux.com>, 
	Rob Herring <robh@kernel.org>, Bjorn Helgaas <bhelgaas@google.com>, 
	Marek Vasut <marek.vasut+renesas@gmail.com>, Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>, 
	Kishon Vijay Abraham I <kishon@kernel.org>, linux-pci@vger.kernel.org, linux-kernel@vger.kernel.org, 
	linux-renesas-soc@vger.kernel.org, linux-arm-msm@vger.kernel.org, mhi@lists.linux.dev
Subject: Re: [PATCH v3 1/5] PCI: dwc: Refactor dw_pcie_edma_find_chip() API
Message-ID: <6r7kquumuaga5j2hosyi6fla6frdzm5e4iobt7dtftjuwm7wku@7wij7dfhneob>
References: <20240226-dw-hdma-v3-0-cfcb8171fc24@linaro.org>
 <20240226-dw-hdma-v3-1-cfcb8171fc24@linaro.org>
 <fielxplkgrvz5qmqrrq5ahmah5yqx7anjylrlcqyev2z2cl2wo@3ltyl242vkba>
 <20240226152757.GF8422@thinkpad>
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In-Reply-To: <20240226152757.GF8422@thinkpad>

On Mon, Feb 26, 2024 at 08:57:57PM +0530, Manivannan Sadhasivam wrote:
> On Mon, Feb 26, 2024 at 03:45:16PM +0300, Serge Semin wrote:
> > Hi Manivannan
> > 
> > On Mon, Feb 26, 2024 at 05:07:26PM +0530, Manivannan Sadhasivam wrote:
> > > In order to add support for Hyper DMA (HDMA), let's refactor the existing
> > > dw_pcie_edma_find_chip() API by moving the common code to separate
> > > functions.
> > > 
> > > No functional change.
> > > 
> > > Suggested-by: Serge Semin <fancer.lancer@gmail.com>
> > > Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
> > > ---
> > >  drivers/pci/controller/dwc/pcie-designware.c | 52 +++++++++++++++++++++-------
> > >  1 file changed, 39 insertions(+), 13 deletions(-)
> > > 
> > > diff --git a/drivers/pci/controller/dwc/pcie-designware.c b/drivers/pci/controller/dwc/pcie-designware.c
> > > index 250cf7f40b85..193fcd86cf93 100644
> > > --- a/drivers/pci/controller/dwc/pcie-designware.c
> > > +++ b/drivers/pci/controller/dwc/pcie-designware.c
> > > @@ -880,7 +880,17 @@ static struct dw_edma_plat_ops dw_pcie_edma_ops = {
> > >  	.irq_vector = dw_pcie_edma_irq_vector,
> > >  };
> > >  
> > > -static int dw_pcie_edma_find_chip(struct dw_pcie *pci)
> > > +static void dw_pcie_edma_init_data(struct dw_pcie *pci)
> > > +{
> > > +	pci->edma.dev = pci->dev;
> > > +
> > > +	if (!pci->edma.ops)
> > > +		pci->edma.ops = &dw_pcie_edma_ops;
> > > +
> > > +	pci->edma.flags |= DW_EDMA_CHIP_LOCAL;
> > > +}
> > > +
> > > +static int dw_pcie_edma_find_mf(struct dw_pcie *pci)
> > >  {
> > >  	u32 val;
> > >  
> > > @@ -900,24 +910,27 @@ static int dw_pcie_edma_find_chip(struct dw_pcie *pci)
> > >  	else
> > >  		val = dw_pcie_readl_dbi(pci, PCIE_DMA_VIEWPORT_BASE + PCIE_DMA_CTRL);
> > > 
> > 
> > > -	if (val == 0xFFFFFFFF && pci->edma.reg_base) {
> > > -		pci->edma.mf = EDMA_MF_EDMA_UNROLL;
> > > -
> > > -		val = dw_pcie_readl_dma(pci, PCIE_DMA_CTRL);
> > > -	} else if (val != 0xFFFFFFFF) {
> > > -		pci->edma.mf = EDMA_MF_EDMA_LEGACY;
> > > +	/* Set default mapping format here and update it below if needed */
> > > +	pci->edma.mf = EDMA_MF_EDMA_LEGACY;
> > >  
> > > +	if (val == 0xFFFFFFFF && pci->edma.reg_base)
> > > +		pci->edma.mf = EDMA_MF_EDMA_UNROLL;
> > > +	else if (val != 0xFFFFFFFF)
> > >  		pci->edma.reg_base = pci->dbi_base + PCIE_DMA_VIEWPORT_BASE;
> > > -	} else {
> > > +	else
> > >  		return -ENODEV;
> > > -	}
> > 
> > Sorry for not posting my opinion about this earlier, but IMO v2 code
> > was more correct than this one. This version makes the code being not
> > linear as it was in v2, thus harder to comprehend:
> > 
> > 1. Setting up a default value and then overriding it or not makes the
> > reader to keep in mind the initialized value which is harder than to
> > just read what is done in the respective branch.
> > 
> 
> No, I disagree. Whether we set the default value or not, EDMA_MF_EDMA_LEGACY is
> indeed the default mapping format (this is one of the reasons why the enums
> should start from 1 instead of 0). So initializing it to legacy is not changing
> anything, rather making it explicit.
> 
> > 2. Splitting up the case clause with respective inits and the mapping
> > format setting up also makes it harder to comprehend what's going on.
> > In the legacy case the reg-base address and the mapping format init are
> > split up while they should have been done simultaneously only if (val
> > != 0xFFFFFFFF).
> > 
> 
> Well again, this doesn't matter since the default mapping format is legacy. But
> somewhat agree that the two clauses are setting different fields, but even if
> the legacy mapping format is set inside the second clause, it still differs from
> the first one since we are not setting reg_base.
> 
> > 3. The most of the current devices has the unrolled mapping (available
> > since v4.9 IP-core), thus having the mf field pre-initialized produces
> > a redundant store operation for the most of the modern devices.
> > 
> 
> Ok, this one I agree. We could avoid the extra assignment.
> 
> > 4. Getting rid from the curly braces isn't something what should be
> > avoided at any cost and doesn't give any optimization really. It
> > doesn't cause having less C-lines of the source code and doesn't
> > improve the code readability.
> > 
> 
> Yeah, there is no benefit other than a simple view of the code. But for point
> (3), I agree to roll back to v2 version.
> 
> > So to speak, I'd suggest to get back the v2 implementation here.
> > 
> > >  
> > > -	pci->edma.dev = pci->dev;
> > > +	return 0;
> > > +}
> > >  
> > > -	if (!pci->edma.ops)
> > > -		pci->edma.ops = &dw_pcie_edma_ops;
> > > +static int dw_pcie_edma_find_channels(struct dw_pcie *pci)
> > > +{
> > > +	u32 val;
> > >  
> > > -	pci->edma.flags |= DW_EDMA_CHIP_LOCAL;
> > 
> > > +	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);
> > 
> > Just dw_pcie_readl_dma(pci, PCIE_DMA_CTRL)
> > 
> 
> 'val' is uninitialized. Why should the assignment be skipped?

The entire

+	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);

can be replaced with a single line

+	val = dw_pcie_readl_dma(pci, PCIE_DMA_CTRL);

since in the legacy case (reg_base = PCIE_DMA_VIEWPORT_BASE) and the
reg_base has been initialized by now.

-Serge(y)

> 
> - Mani
> 
> > -Serge(y)
> > 
> > >  
> > >  	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);
> > > @@ -930,6 +943,19 @@ static int dw_pcie_edma_find_chip(struct dw_pcie *pci)
> > >  	return 0;
> > >  }
> > >  
> > > +static int dw_pcie_edma_find_chip(struct dw_pcie *pci)
> > > +{
> > > +	int ret;
> > > +
> > > +	dw_pcie_edma_init_data(pci);
> > > +
> > > +	ret = dw_pcie_edma_find_mf(pci);
> > > +	if (ret)
> > > +		return ret;
> > > +
> > > +	return dw_pcie_edma_find_channels(pci);
> > > +}
> > > +
> > >  static int dw_pcie_edma_irq_verify(struct dw_pcie *pci)
> > >  {
> > >  	struct platform_device *pdev = to_platform_device(pci->dev);
> > > 
> > > -- 
> > > 2.25.1
> > > 
> 
> -- 
> மணிவண்ணன் சதாசிவம்