linux-kernel - Re: [PATCH v2 4/6] PCI: brcmstb: add Broadcom STB PCIe host controller driver

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Message-ID: <24e5302a-51c6-df39-5381-a790752f261d@arm.com>
Date:   Fri, 22 Nov 2019 14:59:06 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Jim Quinlan <jim2101024@...il.com>,
        Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
Cc:     Florian Fainelli <f.fainelli@...il.com>, maz@...nel.org,
        Phil Elwell <phil@...pberrypi.org>,
        linux-kernel@...r.kernel.org,
        Jeremy Linton <jeremy.linton@....com>,
        Eric Anholt <eric@...olt.net>, mbrugger@...e.com,
        bcm-kernel-feedback-list <bcm-kernel-feedback-list@...adcom.com>,
        Stefan Wahren <wahrenst@....net>,
        Jim Quinlan <james.quinlan@...adcom.com>,
        linux-pci <linux-pci@...r.kernel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Andrew Murray <andrew.murray@....com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        "moderated list:ARM/FREESCALE IMX / MXC ARM ARCHITECTURE" 
        <linux-arm-kernel@...ts.infradead.org>,
        linux-rpi-kernel@...ts.infradead.org
Subject: Re: [PATCH v2 4/6] PCI: brcmstb: add Broadcom STB PCIe host
 controller driver

On 21/11/2019 9:07 pm, Jim Quinlan wrote:
[...]
>> As for [...]_NUM_MASK_BITS I'm looking for a smart/generic way to calculate it
>> from the actual mask. No luck so far. If not, I think I'll simply leave it as
>> is for now.

HWEIGHT()?

>>>> FYI, What's happening here is that we have to save the CPU address range
>>>> (which
>>>> is already shifted right 20 positions) in two parts, the lower 12 bits go
>>>> into
>>>> PCIE_MISC_CPU_2_PCIE_MEM_WIN0_BASE_LIMIT while the higher 8 bits go into
>>>> PCIE_MISC_CPU_2_PCIE_MEM_WIN0_BASE_HI or
>>>> PCIE_MISC_CPU_2_PCIE_MEM_WIN0_LIMIT_HI.
>>>
>>> The hardware spec require bits 31:20 of the address, and the high registers
>>> require 39:32 right?
>>
>> Yes, that's it.
>>
>>> (Apologies, the indirection by the WR_FLD_** macros easily confuses me. These
>>> type of macros are helpful, or rather would be if the whole kernel used them.
>>> I think they can add confusion when each driver has its own set of similar
>>> macros. This is why its *really* helpful to use any existing macros in the
>>> kernel - and only invent new ones if needed).
>>
>> I agree it's pretty confusing, I think v3, using bitfield.h as much as
>> possible, looks substantially more welcoming.
> 
> The reason we use custom macros is because we'd like to keep the
> register names the same as the HW declares and our internal tools
> support.  As you may have noticed, our register names are unusually
> long and it is hard to fit a simple read or write field assignment
> within 80 columns w/o using custom macros tailored to our register
> names' format.
> 
> Perhaps Nicolas can pull a rabbit out of a hat and use Linux macros
> while keeping our long register names, but if he has to use his own
> shorter register names it will become harder for Broadcom developers
> to debug this driver.

Regardless of the length of the names, the standard bitfield helpers can 
still make things easier to reason about - in this particular case I 
think you could lose some boilerplate and indirection with essentially 
no change to the readability you're concerned for - compare:

#define REG_NAME ...
#define REG_NAME_FIELD_NAME_MASK ...
#define REG_NAME_FIELD_NAME_SHIFT ...

	val = RD_FIELD(base, REG_NAME,
		       FIELD_NAME);

vs.

#define REG_NAME ...
#define   FIELD_NAME ...

	reg = bcm_readl(base + REG_NAME);
	val = FIELD_GET(FIELD_NAME, reg);

Robin.