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

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Message-ID: <cc958b0c-b947-2b3f-9e4b-9216e0085e6e@free.fr>
Date:   Mon, 27 Mar 2017 21:44:08 +0200
From:   Mason <slash.tmp@...e.fr>
To:     Marc Zyngier <marc.zyngier@....com>,
        Bjorn Helgaas <helgaas@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     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 27/03/2017 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?

Regards.