linux-kernel - Re: [PATCH v2 2/9] PCI: keystone: Modify legacy_irq_handler to check the IRQ

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Date:   Fri, 8 Feb 2019 10:02:05 +0530
From:   Kishon Vijay Abraham I <kishon@...com>
To:     Lorenzo Pieralisi <lorenzo.pieralisi@....com>
CC:     Murali Karicheri <m-karicheri2@...com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Jingoo Han <jingoohan1@...il.com>,
        Gustavo Pimentel <gustavo.pimentel@...opsys.com>,
        <linux-pci@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 2/9] PCI: keystone: Modify legacy_irq_handler to check
 the IRQ_STATUS of INTA/B/C/D

Hi Lorenzo,

On 07/02/19 9:45 PM, Lorenzo Pieralisi wrote:
> On Thu, Feb 07, 2019 at 04:39:17PM +0530, Kishon Vijay Abraham I wrote:
>> The legacy interrupt handler directly checks the IRQ_STATUS register
>> corresponding to a interrupt line inorder to invoke generic_handle_irq.
>>
>> While this is okay for K2G platform which has separate interrupt line for
>> each of the 4 legacy interrupts, AM654 which uses the same PCIe wrapper
>> has a single interrupt line for all the legacy interrupts. So for AM654
>> the interrupt handler won't be able to directly check the IRQ_STATUS
>> register corresponding to the interrupt line.
>>
>> Also the legacy interrupt handler uses 'virq' obtained from
>> irq_of_parse_and_map to find the correct interrupt line which raised the
>> interrupt. There is no guarantee that virq assigned for contiguous hardware
>> irq will be contiguous and the interrupt handler might end up checking
>> the wrong IRQ_STATUS register.
>>
>> In order to overcome the above issues, read the IRQ_STATUS register of
>> all the 4 legacy interrupts to determine which interrupt was raised.
>>
>> Link: https://lkml.kernel.org/r/bb081d21-7c03-0357-4294-7e92d95d838c@arm.com
>> Signed-off-by: Kishon Vijay Abraham I <kishon@...com>
>> ---
>>  drivers/pci/controller/dwc/pci-keystone.c | 22 ++++++++++++----------
>>  1 file changed, 12 insertions(+), 10 deletions(-)
>>
>> diff --git a/drivers/pci/controller/dwc/pci-keystone.c b/drivers/pci/controller/dwc/pci-keystone.c
>> index 5286a480f76b..4cf9849d5a1d 100644
>> --- a/drivers/pci/controller/dwc/pci-keystone.c
>> +++ b/drivers/pci/controller/dwc/pci-keystone.c
>> @@ -214,16 +214,11 @@ static void ks_pcie_handle_legacy_irq(struct keystone_pcie *ks_pcie,
>>  {
>>  	struct dw_pcie *pci = ks_pcie->pci;
>>  	struct device *dev = pci->dev;
>> -	u32 pending;
>>  	int virq;
>>  
>> -	pending = ks_pcie_app_readl(ks_pcie, IRQ_STATUS(offset));
>> -
>> -	if (BIT(0) & pending) {
>> -		virq = irq_linear_revmap(ks_pcie->legacy_irq_domain, offset);
>> -		dev_dbg(dev, ": irq: irq_offset %d, virq %d\n", offset, virq);
>> -		generic_handle_irq(virq);
>> -	}
>> +	virq = irq_linear_revmap(ks_pcie->legacy_irq_domain, offset);
>> +	dev_dbg(dev, ": irq: irq_offset %d, virq %d\n", offset, virq);
>> +	generic_handle_irq(virq);
>>  
>>  	/* EOI the INTx interrupt */
>>  	ks_pcie_app_writel(ks_pcie, IRQ_EOI, offset);
>> @@ -607,8 +602,9 @@ static void ks_pcie_legacy_irq_handler(struct irq_desc *desc)
>>  	struct keystone_pcie *ks_pcie = irq_desc_get_handler_data(desc);
>>  	struct dw_pcie *pci = ks_pcie->pci;
>>  	struct device *dev = pci->dev;
>> -	u32 irq_offset = irq - ks_pcie->legacy_host_irqs[0];
>>  	struct irq_chip *chip = irq_desc_get_chip(desc);
>> +	unsigned int irq_no;
>> +	u32 reg;
>>  
>>  	dev_dbg(dev, ": Handling legacy irq %d\n", irq);
>>  
>> @@ -618,7 +614,13 @@ static void ks_pcie_legacy_irq_handler(struct irq_desc *desc)
>>  	 * ack operation.
>>  	 */
>>  	chained_irq_enter(chip, desc);
>> -	ks_pcie_handle_legacy_irq(ks_pcie, irq_offset);
>> +	for (irq_no = 0; irq_no < PCI_NUM_INTX; irq_no++) {
> 
> I understand the aim of this code but now on platforms where there
> is a 1:1 relationship between Linux IRQ and INTX this loop has
> steps carried out for nothing.
> 
> If I understand the code correctly what this code does is force
> looping over INTX status regs regardless of what linux IRQ number was
> actually active.

right. This driver is used by 2 platforms K2G and AM654 (The patches are there
on the list). K2G has 4 interrupt lines for each of the 4 legacy interrups
while AM654 has a single interrupt line. One of the purpose of this patch is to
have a single legacy interrupt handler for both K2G and AM654.
> 
> You could do something faster by creating a matrix LinuxIRQ x INTx to
> detect what INTx status register should actually be checked.
> 
> This seems overkill to me but it is not that complicated to implement
> and may clarify the code (and avoid reading up to three registers for
> nothing on the IRQ code path, which can make things faster too).

Agreed. But that's not possible for AM654 which has a single interrupt line and
all the registers has to be read to identify the interrupt source.

Thanks
Kishon