linux-kernel - Re: [PATCH v10 5/7] PCI: mediatek-gen3: Add MSI support

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Date:   Tue, 20 Apr 2021 13:08:32 +0200
From:   Pali Rohár <pali@...nel.org>
To:     Marc Zyngier <maz@...nel.org>
Cc:     Jianjun Wang <jianjun.wang@...iatek.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Rob Herring <robh+dt@...nel.org>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Ryder Lee <ryder.lee@...iatek.com>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        Matthias Brugger <matthias.bgg@...il.com>,
        linux-pci@...r.kernel.org, linux-mediatek@...ts.infradead.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, youlin.pei@...iatek.com,
        chuanjia.liu@...iatek.com, qizhong.cheng@...iatek.com,
        sin_jieyang@...iatek.com, drinkcat@...omium.org,
        Rex-BC.Chen@...iatek.com, anson.chuang@...iatek.com,
        Krzysztof Wilczyski <kw@...ux.com>
Subject: Re: [PATCH v10 5/7] PCI: mediatek-gen3: Add MSI support

On Tuesday 20 April 2021 12:01:10 Marc Zyngier wrote:
> On Tue, 20 Apr 2021 10:44:02 +0100,
> Pali Rohár <pali@...nel.org> wrote:
> > 
> > Hello!
> > 
> > On Tuesday 20 April 2021 14:17:21 Jianjun Wang wrote:
> > > +static void mtk_pcie_enable_msi(struct mtk_pcie_port *port)
> > > +{
> > > +	int i;
> > > +	u32 val;
> > > +
> > > +	for (i = 0; i < PCIE_MSI_SET_NUM; i++) {
> > > +		struct mtk_msi_set *msi_set = &port->msi_sets[i];
> > > +
> > > +		msi_set->base = port->base + PCIE_MSI_SET_BASE_REG +
> > > +				i * PCIE_MSI_SET_OFFSET;
> > > +		msi_set->msg_addr = port->reg_base + PCIE_MSI_SET_BASE_REG +
> > > +				    i * PCIE_MSI_SET_OFFSET;
> > > +
> > > +		/* Configure the MSI capture address */
> > > +		writel_relaxed(lower_32_bits(msi_set->msg_addr), msi_set->base);
> > > +		writel_relaxed(upper_32_bits(msi_set->msg_addr),
> > > +			       port->base + PCIE_MSI_SET_ADDR_HI_BASE +
> > > +			       i * PCIE_MSI_SET_ADDR_HI_OFFSET);
> > 
> > This looks like as setting MSI doorbell address to MSI doorbell address.
> > 
> > > +static void mtk_compose_msi_msg(struct irq_data *data, struct msi_msg *msg)
> > > +{
> > > +	struct mtk_msi_set *msi_set = irq_data_get_irq_chip_data(data);
> > > +	struct mtk_pcie_port *port = data->domain->host_data;
> > > +	unsigned long hwirq;
> > > +
> > > +	hwirq =	data->hwirq % PCIE_MSI_IRQS_PER_SET;
> > > +
> > > +	msg->address_hi = upper_32_bits(msi_set->msg_addr);
> > > +	msg->address_lo = lower_32_bits(msi_set->msg_addr);
> > > +	msg->data = hwirq;
> > > +	dev_dbg(port->dev, "msi#%#lx address_hi %#x address_lo %#x data %d\n",
> > > +		hwirq, msg->address_hi, msg->address_lo, msg->data);
> > 
> > ... which is later used in compose_msi_msg().
> > 
> > Marc in some other patches for other pci controller drivers changed this
> > address to just main "port" structure. It simplified implementations and
> > also avoided need to declare additional member "msg_addr".
> > 
> > Marc, would it be possible to simplify it also for this driver and just
> > set msg_addr to virt_to_phys(port)?
> 
> Maybe. It really depends on what range the HW accepts, and the sole
> requirement is to use an address that the endpoint cannot DMA
> to. Here, the driver seems to be using something based on the port
> base address, which is good enough as far as I am concerned (the thing
> I usually object to is the allocation of memory just for the sake of
> getting a capture address).
> 
> If you want to further simplify it, you could simply use port.reg_base
> as the MSI address for all sets, as I don't think they have to be
> distinct. But someone with access to the TRM for this should go and
> check it.
> 
> I don't believe this should gate the merging od this driver though.

Of course! I'm just asking details to understand best practises and how
it works. So thanks for information!

> 	M.
> 
> -- 
> Without deviation from the norm, progress is not possible.