On Mon, 26 May 2025 at 12:13, Sughosh Ganu <sughosh.g...@linaro.org> wrote: > > The LMB module has undergone significant changes in the recent > past. Add a document which briefly describes what the LMB module does, > and the changes that have been made to it's design since the 2025.01 > release. > > Signed-off-by: Sughosh Ganu <sughosh.g...@linaro.org>
Acked-by: Ilias Apalodimas <ilias.apalodi...@linaro.org> > --- > Changes since V2: > * Reflect the changed name of the API (lmb_alloc_mem) > > doc/api/index.rst | 1 - > doc/api/lmb.rst | 7 -- > doc/develop/index.rst | 1 + > doc/develop/lmb.rst | 166 ++++++++++++++++++++++++++++++++++++++++++ > 4 files changed, 167 insertions(+), 8 deletions(-) > delete mode 100644 doc/api/lmb.rst > create mode 100644 doc/develop/lmb.rst > > diff --git a/doc/api/index.rst b/doc/api/index.rst > index 506843ed74a..cf9d21e4c1c 100644 > --- a/doc/api/index.rst > +++ b/doc/api/index.rst > @@ -17,7 +17,6 @@ U-Boot API documentation > interrupt > led > linker_lists > - lmb > logging > nvmem > part > diff --git a/doc/api/lmb.rst b/doc/api/lmb.rst > deleted file mode 100644 > index 2095bfa1618..00000000000 > --- a/doc/api/lmb.rst > +++ /dev/null > @@ -1,7 +0,0 @@ > -.. SPDX-License-Identifier: GPL-2.0+ > - > -Logical memory blocks > -===================== > - > -.. kernel-doc:: include/lmb.h > - :internal: > diff --git a/doc/develop/index.rst b/doc/develop/index.rst > index 0c83ef109ab..3c044e67927 100644 > --- a/doc/develop/index.rst > +++ b/doc/develop/index.rst > @@ -46,6 +46,7 @@ Implementation > cedit > event > global_data > + lmb > logging > makefiles > menus > diff --git a/doc/develop/lmb.rst b/doc/develop/lmb.rst > new file mode 100644 > index 00000000000..b9d0f09c2bb > --- /dev/null > +++ b/doc/develop/lmb.rst > @@ -0,0 +1,166 @@ > +.. SPDX-License-Identifier: GPL-2.0+ > + > +Logical Memory Blocks (LMB) > +=========================== > + > +U-Boot has support for reserving chunks of memory which is primarily > +used for loading images to the DRAM memory, before these are booted, > +or written to non-volatile storage medium. This functionality is > +provided through the Logical Memory Blocks (LMB) module. > + > +Introduction > +------------ > + > +The LMB module manages allocation requests for memory region not > +occupied by the U-Boot image. Allocation requests that are made > +through malloc() and similar functions result in memory getting > +allocated from the heap region, which is part of the U-Boot > +image. Typically, the heap memory is a few MiB in size. Loading an > +image like the linux kernel might require lot more memory than what > +the heap can provide. Such allocations are usually handled through the > +LMB module. > + > +The U-Boot image typically gets relocated to the top of the usable > +DRAM memory region. A typical memory layout looks as follows:: > + > + > + > + > + > + | | > + | | > + | | > + | | > + | | > + --- +--------------+ <--- U-Boot ram top > + | | | > + | | Text | > + | +--------------+ > + | | | > + | | Data | > + | +--------------+ > + | | | > + | | BSS | > + U-Boot Image +--------------+ > + | | | > + | | Heap | > + | | | > + | +--------------+ > + | | | > + | | | > + | | Stack | > + | | | > + | | | > + --- +--------------+ > + | | > + | | > + | | > + | | > + | | > + | | > + | | > + | | > + | | > + | | > + | | > + | | > + | | > + | | > + +--------------+ <--- ram start > + > + > + > +The region of memory below the U-Boot image is the one controlled by > +the LMB module. > + > + > +Types of LMB Allocations > +------------------------ > + > +There are two classes of allocation requests that get made to the LMB > +module. One type of allocation requests are requesting memory of a > +particular number of bytes. This type of allocation is similar to that > +done using the malloc type of function calls. The other type of > +allocations, are requests made for a specific memory address. The > +second type of allocations are usually made for loading images to a > +particular memory address. > + > + > +LMB design Pre 2025.01 > +---------------------- > + > +The earlier versions of U-Boot (pre 2025.01 release) > +had a local memory map based LMB implementation whereby it was > +possible to declare the LMB map inside a function or a C file. This > +design resulted in temporary, non-global LMB maps, which also allowed > +for re-use of memory. This meant that it was possible to use a region > +of memory to load some image, and subsequently the same region of > +memory could be used for loading a different image. A typical example > +of this usage would be loading an image to a memory address, followed > +by writing that image to some non-volatile storage medium. Once this > +is done, the same address can be used for loading a different image > +and then writing it to it's non-volatile storage > +destination. Typically, environment variables like `loadaddr`, > +`kernel_addr_r`, `ramdisk_addr_r` are used for loading images to > +memory regions. > + > + > +Current LMB implementation > +-------------------------- > + > +Changes were made in the 2025.01 release to make the LMB memory map > +global and persistent. With this, the LMB memory map is the same > +across all of U-Boot, and also persists as long as U-Boot is > +active. Even with this change, there has been consistency as far as > +re-use of memory is concerned to maintain backward compatibility. It > +is allowed for re-requesting the same region of memory if the memory > +region has a particular attribute (LMB_NONE). > + > +As part of the platform boot, DRAM memory available for use in U-Boot > +gets added to the LMB memory map. Any allocation requests made > +subsequently will be made from this memory added as part of the board > +init. > + > + > +Allocation API > +-------------- > + > +Any request for non-heap memory can be made through the LMB allocation > +API. > + > +.. code-block:: c > + > + int lmb_alloc_mem(enum lmb_mem_type type, u64 align, > + phys_addr_t *addr, phys_size_t size, > + u32 flags); > + > +Correspondingly, the allocated memory can be free'd > + > +.. code-block:: c > + > + long lmb_free(phys_addr_t base, phys_size_t size, u32 flags); > + > +For a detailed API description, please refer to the header file. > + > + > +UEFI allocations with LMB as the backend > +---------------------------------------- > + > +The UEFI specification describes boot-time API's for allocation of > +memory. These API's use the same memory that is being used by the LMB > +module. Pre 2025.01 release, there wasn't any synchronisation between > +the EFI sub-system and the LMB module about the memory that was > +getting allocated by each of these modules. This was the primary > +reason for making the LMB memory map global and persistent. With this > +change, the EFI memory allocation API's have also been changed to use > +the LMB module as the backend for the allocation requests. Any other > +sub-system which might wish to use the same memory region for it's use > +can then use the LMB as the backend for the memory allocations and > +it's associated book-keeping. > + > + > +API documentation > +----------------- > + > +.. kernel-doc:: include/lmb.h > + > -- > 2.34.1 >
