linux-kernel - Re: [RFC PATCH v0.2] PCI: Add support for tango PCIe host bridge

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Message-ID: <05801ad3-23b0-be04-883c-a018fd5df6f8@arm.com>
Date:   Tue, 28 Mar 2017 09:21:51 +0100
From:   Marc Zyngier <marc.zyngier@....com>
To:     Mason <slash.tmp@...e.fr>
Cc:     Bjorn Helgaas <helgaas@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Robin Murphy <robin.murphy@....com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Liviu Dudau <liviu.dudau@....com>,
        David Laight <david.laight@...lab.com>,
        linux-pci <linux-pci@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Thibaud Cornic <thibaud_cornic@...madesigns.com>,
        Phuong Nguyen <phuong_nguyen@...madesigns.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH v0.2] PCI: Add support for tango PCIe host bridge

On 27/03/17 23:04, Mason wrote:
> On 27/03/2017 23:07, Marc Zyngier wrote:
> 
>> On Mon, Mar 27 2017 at 08:44:08 PM, Mason wrote:
>>
>>> On 27/03/2017 19:09, Marc Zyngier wrote:
>>>
>>>> On 27/03/17 16:53, Mason wrote:
>>>>
>>>>> I have one remaining issue with bitmaps.
>>>>>
>>>>> My HW regs are 32b. How do I grab e.g. bits 96-127?
>>>>> All I can think of is
>>>>>   u32 val = ((u32 *)bitmap)[3];
>>>>>
>>>>> Is this acceptable?
>>>>
>>>> No.
>>>>
>>>>> mrutland mentioned bitmap_to_u32array() but IIUC it is used to
>>>>> copy an entire bitmap.
>>>>
>>>> The real question is "Why do you need such a thing?".
>>>
>>> You told me to use an in-memory version of the "unmasked"
>>> bitmap, yes? In that case, I need to be able to grab
>>> a piece of said bitmap, to update the corresponding piece
>>> of the HW bitmap.
>>>
>>> For example, assume the first 3 MSIs are unmasked,
>>> in other words, unmasked = 0x7
>>>
>>> A new user comes along and wants to assign an MSI.
>>> Scan "unmasked", found bit at pos 3.
>>> Update "unmasked" to 0xf.
>>> At some point, I need to write 0xf to some HW reg.
>>> So I need to grab a piece of "unmasked" (bits 0-31 in my example)
>>>
>>>   pos = find_first_zero_bit(unmasked, COUNT);
>>>   __set_bit(pos, unmasked);
>>>   int reg_index = pos / 32;
>>>   u32 val = ((u32 *)unmasked)[reg_index];
>>>   writel_relaxed(val, pcie->enabled + reg_index * 4);
>>>
>>> Or did I miss something in your suggestion?
>>
>> I don't know, I'm sightly taken aback by your question. Completely
>> puzzled, actually. "Read Modify Write" is a fairly obvious construct.
>>
>>      val = readl_relaxed(pcie->enabled + reg_index);
>>      writel_relaxed(val | BIT(pos % 32), pcie->enabled + reg_index);
>>
>> I never realized this could be such a novel concept. Replace pos with
>> hwirq, add a spinlock, and that's your irq_unmask.
> 
> I'm not sure why you would think I'm not familiar with RMW.
> 
> My original code was:
> 
> 	mask = readl_relaxed(pcie->msi_mask);
> 	pos = find_first_zero_bit(&mask, 32);
> 	writel(mask | BIT(pos), pcie->msi_mask);
> 
> To which you replied:
> "Do you really need to read this from the HW each time you allocate an
> interrupt? That feels pretty crazy. You're much better off having an
> in-memory bitmap that will make things more efficient"
> 
> This is why I was trying to avoid a MMIO read from the HW
> each time I allocate an interrupt.

Yes, and I stand by this comment. I also said, in the bit that you
didn't quote, that the RMW was to be moved to the irq_unmask method.
This ensures separation of IRQ allocation and IRQ chip operations.

>> My suggestion was to use a bitmap in order not to perform extra MMIO
>> accesses on the fast path. It doesn't mean that you cannot read from the
>> register under any circumstances. It just means that you don't do it
>> when there are more efficient ways to do it.
> 
> The fast path is the interrupt handler, right?
> (I didn't read the interrupt mask in the ISR.)

You didn't then, and yet you have to, since you need to support more
than 32 MSIs, requiring you to either read these mask regs in the HW, or
(tata!) look into the allocation bitmap.

	M.
-- 
Jazz is not dead. It just smells funny...