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

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Date:   Fri, 24 Mar 2017 00:40:16 +0100
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 23/03/2017 18:03, Mason wrote:

> The host bridge actually supports 256 MSIs.
> 
> IIUC, what you suggested on IRC is that I support 256 in the driver,
> and only read the status for *enabled* MSIs.
> 
> Pseudo-code:
> 
> for every 32-bit blob in the enabled bitmap
>   if the value is non-zero
>     lookup the corresponding status reg
> 
> Problem is that a BITMAP is unsigned long (as you point out below).
> So I'm not sure how to iterate 32-bits at a time over the BITMAP.

Something along these lines:

	DECLARE_BITMAP(enabled, 256);

	unsigned int pos = 0;

	while ((pos = find_next_bit(enabled, 256, pos)) < 256) {
		int offset = (pos / 32) * 4;
		u32 status = readl_relaxed(status + offset);
		/* Handle each pos set in status */
		pos = round_up(pos, 32);
	}

You mentioned a bug in my code (due to the platform endianness)
when passing the result of readl_relaxed to the bitops routine...
How is one supposed to iterate over status?

I'm not yet seeing this endianness issue, since (status & BIT(i))
provides the status of MSI_i, irrespective of endianness.

Although I see that arch/arm/include/asm/bitops.h declares
BE and LE variants... I'm confused.

Regards.