linux-kernel - Re: [RFC PATCH 2/2] mm/pmem: Add memblock based e820 platform driver

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Date:   Fri, 6 Jul 2018 11:46:48 -0700
From:   Randy Dunlap <rdunlap@...radead.org>
To:     "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        akpm@...ux-foundation.org, Dan Williams <dan.j.williams@...el.com>,
        Oliver <oohall@...il.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 2/2] mm/pmem: Add memblock based e820 platform driver

On 07/06/18 01:29, Aneesh Kumar K.V wrote:
> This patch steal system RAM and use that to emulate pmem device using the
> e820 platform driver.
> 
> This adds a new kernel command line 'pmemmap' which takes the format <size[KMG]>
> to allocate memory early in the boot. This memory is later registered as
> persistent memory range.
> 
> Based on original patch from Oliver OHalloran <oliveroh@....ibm.com>
> 
> Not-Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@...ux.ibm.com>
> ---
>  drivers/nvdimm/Kconfig        |  13 ++++
>  drivers/nvdimm/Makefile       |   1 +
>  drivers/nvdimm/memblockpmem.c | 115 ++++++++++++++++++++++++++++++++++
>  3 files changed, 129 insertions(+)
>  create mode 100644 drivers/nvdimm/memblockpmem.c
> 
> diff --git a/drivers/nvdimm/Kconfig b/drivers/nvdimm/Kconfig
> index 50d2a33de441..cbbbcbd4506b 100644
> --- a/drivers/nvdimm/Kconfig
> +++ b/drivers/nvdimm/Kconfig
> @@ -115,4 +115,17 @@ config OF_PMEM
>  config PMEM_PLATFORM_DEVICE
>         bool
>  
> +config MEMBLOCK_PMEM
> +	bool "pmemmap= parameter support"
> +	default y
> +	depends on HAVE_MEMBLOCK
> +	select PMEM_PLATFORM_DEVICE
> +	help
> +	  Add support for the pmemmap= kernel command line parameter. This is similar
> +	  to the memmap= parameter available on ACPI platforms, but it uses generic
> +	  kernel facilities (the memblock allocator) to reserve memory rather than adding
> +	  to the e820 table.
> +
> +	  Select Y if unsure.
> +
>  endif


There's a high barrier for "default y", something like if the platform or device
cannot boot without it, it can be "default y".  I have doubts that this is OK.


-- 
~Randy